Issue title: reports of crashes when importing wallet backup while blockchain is not re-synced
 Issue body: and people don't realize your balance won't show, etc. need to address this

 Comments: 
 Comment 0: Related to: #514

 Comment 1: How can we fix this?  I suggest on mainnet, disallow importing wallet backups when the chain is out of date until the crashing and UI issues are resolved.

We need to investigate the crashes.  I'm thinking I'll try to see if I can reproduce the crash with testnet or unit tests.

 Comment 2: I'm not sure exactly what this issue is referring to, but there was a bug of about this vintage where importing a wallet while syncing crashed.  This was caused by:
- wallet starts to run.  after some time, it calls 38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5yield() to allow other tasks to run
- a new block comes in from the network.  
- chain_db notifies wallet that there's a new block
- wallet starts scanning the new block, not realizing it was already in the process of scanning.

f80db8fb4ac358038d40cd7a70d102c841608639 and 570869a3789109471bea77878eac3e5926ef8aa3 may be fixes for the crash (although I seem to recall Dan L had a hand in the fix).

I don't have a clue about the "and people don't realize your balance won't show" part of this issue though.

 Comment 3: The only way to deal with balances not showing during syncing is to move to a thin-client style architecture where your balances and keys are stored on a server for you somewhere. I'd say that's not a serious issue for full nodes.

 Comment 4: Okay.  I think this issue is closeable, then.

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