Issue title: db_plugin not storing blocks
 Issue body: I am testing the db_plugin to store the chain data in mongo.

The EOS database was created and there are Blocks, Messages and Accounts collections.  Accounts contains 1 document with name "eos" but the other 2 collections are empty

The eosd reports this error almost every block

3252004ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

The currency example contract works and persists after an eosd restart
 Comments: 
 Comment 0: The error is an indications that an attempt was made to restart `./eosd` with an existing blockchain and existing mongodb but that no blocks were persisted in the previous run. Can you provide steps to reproduce starting with an empty mongodb and new chain? Please include command line arguments to `eosd`.
 Comment 1: eosd is being launched with this command

LD_LIBRARY_PATH=/usr/local/lib/ ./eosd

I added the LD_LIBRARY for the mongo libraries.

It doesnt seem to be restarting itself, here is a full log

1842007ms thread-0   chain_controller.cpp:216      _push_block          ] initq #28177 @2017-11-06T15:30:42  | 0 trx, 0 pending, exectime_ms=1
1842008ms thread-0   producer_plugin.cpp:214       block_production_loo ] initq generated block #28177 @ 2017-11-06T15:30:42 with 0 trxs  0 pending
1842008ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1845003ms thread-0   chain_controller.cpp:216      _push_block          ] inito #28178 @2017-11-06T15:30:45  | 0 trx, 0 pending, exectime_ms=1
1845004ms thread-0   producer_plugin.cpp:214       block_production_loo ] inito generated block #28178 @ 2017-11-06T15:30:45 with 0 trxs  0 pending
1845004ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1848005ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1848007ms thread-0   chain_controller.cpp:216      _push_block          ] inite #28179 @2017-11-06T15:30:48  | 0 trx, 0 pending, exectime_ms=3
1848007ms thread-0   producer_plugin.cpp:214       block_production_loo ] inite generated block #28179 @ 2017-11-06T15:30:48 with 0 trxs  0 pending
1851003ms thread-0   chain_controller.cpp:216      _push_block          ] initc #28180 @2017-11-06T15:30:51  | 0 trx, 0 pending, exectime_ms=1
1851007ms thread-0   producer_plugin.cpp:214       block_production_loo ] initc generated block #28180 @ 2017-11-06T15:30:51 with 0 trxs  0 pending
1851006ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1854005ms thread-0   chain_controller.cpp:216      _push_block          ] initg #28181 @2017-11-06T15:30:54  | 0 trx, 0 pending, exectime_ms=1
1854008ms thread-0   producer_plugin.cpp:214       block_production_loo ] initg generated block #28181 @ 2017-11-06T15:30:54 with 0 trxs  0 pending
1854008ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1857005ms thread-1   db_plugin.cpp:240             process_irreversible ] FC Exception while processing block unspecified (0)
No blocks found in database

1857006ms thread-0   chain_controller.cpp:216      _push_block          ] initm #28182 @2017-11-06T15:30:57  | 0 trx, 0 pending, exectime_ms=3
1857007ms thread-0   producer_plugin.cpp:214       block_production_loo ] initm generated block #28182 @ 2017-11-06T15:30:57 with 0 trxs  0 pending
1860003ms thread-0   chain_controller.cpp:216      _push_block          ] initj #28183 @2017-11-06T15:31:00  | 0 trx, 0 pending, exectime_ms=0
1860003ms thread-0   producer_plugin.cpp:214       block_production_loo ] initj generated block #28183 @ 2017-11-06T15:31:00 with 0 trxs  0 pending
1863002ms thread-0   chain_controller.cpp:216      _push_block          ] initu #28184 @2017-11-06T15:31:03  | 0 trx, 0 pending, exectime_ms=0
1863002ms thread-0   producer_plugin.cpp:214       block_production_loo ] initu generated block #28184 @ 2017-11-06T15:31:03 with 0 trxs  0 pending
1866003ms thread-0   chain_controller.cpp:216      _push_block          ] initp #28185 @2017-11-06T15:31:06  | 0 trx, 0 pending, exectime_ms=0
1866003ms thread-0   producer_plugin.cpp:214       block_production_loo ] initp generated block #28185 @ 2017-11-06T15:31:06 with 0 trxs  0 pending
1869002ms thread-0   chain_controller.cpp:216      _push_block          ] inita #28186 @2017-11-06T15:31:09  | 0 trx, 0 pending, exectime_ms=0
1869002ms thread-0   producer_plugin.cpp:214       block_production_loo ] inita generated block #28186 @ 2017-11-06T15:31:09 with 0 trxs  0 pending
1872003ms thread-0   chain_controller.cpp:216      _push_block          ] initg #28187 @2017-11-06T15:31:12  | 0 trx, 0 pending, exectime_ms=0
1872003ms thread-0   producer_plugin.cpp:214       block_production_loo ] initg generated block #28187 @ 2017-11-06T15:31:12 with 0 trxs  0 pending
1875002ms thread-0   chain_controller.cpp:216      _push_block          ] initm #28188 @2017-11-06T15:31:15  | 0 trx, 0 pending, exectime_ms=0
1875003ms thread-0   producer_plugin.cpp:214       block_production_loo ] initm generated block #28188 @ 2017-11-06T15:31:15 with 0 trxs  0 pending
 Comment 2: Certainly in a bad state now. If you can provide repro steps to get into this state that would be appreciated.

To get back to a clean state, start with:  `./eosd --resync`

 Comment 3: Just starting eosd with no command line options results in broken state.  Attached is my config.ini

[config.ini.zip](https://github.com/EOSIO/eos/files/1447956/config.ini.zip)

restarting with --resync fixes everything


 Comment 4: I tried your config.ini file and was not able to reproduce your issue. The `./eosd --resync` wipes the database. I suspect you had a previous run with an improper shutdown. If you can provide steps to reproduce your problem feel free to reopen this issue or create another.
 Comment 5: hi,heifner,i have resolved the same issue by your suggestion "./eosd --resync"
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