Replication: fix the infamous key leakage of writable slaves + EXPIRE.

BACKGROUND AND USE CASEj

Redis slaves are normally write only, however the supprot a "writable"
mode which is very handy when scaling reads on slaves, that actually
need write operations in order to access data. For instance imagine
having slaves replicating certain Sets keys from the master. When
accessing the data on the slave, we want to peform intersections between
such Sets values. However we don't want to intersect each time: to cache
the intersection for some time often is a good idea.

To do so, it is possible to setup a slave as a writable slave, and
perform the intersection on the slave side, perhaps setting a TTL on the
resulting key so that it will expire after some time.

THE BUG

Problem: in order to have a consistent replication, expiring of keys in
Redis replication is up to the master, that synthesize DEL operations to
send in the replication stream. However slaves logically expire keys
by hiding them from read attempts from clients so that if the master did
not promptly sent a DEL, the client still see logically expired keys
as non existing.

Because slaves don't actively expire keys by actually evicting them but
just masking from the POV of read operations, if a key is created in a
writable slave, and an expire is set, the key will be leaked forever:

1. No DEL will be received from the master, which does not know about
such a key at all.

2. No eviction will be performed by the slave, since it needs to disable
eviction because it's up to masters, otherwise consistency of data is
lost.

THE FIX

In order to fix the problem, the slave should be able to tag keys that
were created in the slave side and have an expire set in some way.

My solution involved using an unique additional dictionary created by
the writable slave only if needed. The dictionary is obviously keyed by
the key name that we need to track: all the keys that are set with an
expire directly by a client writing to the slave are tracked.

The value in the dictionary is a bitmap of all the DBs where such a key
name need to be tracked, so that we can use a single dictionary to track
keys in all the DBs used by the slave (actually this limits the solution
to the first 64 DBs, but the default with Redis is to use 16 DBs).

This solution allows to pay both a small complexity and CPU penalty,
which is zero when the feature is not used, actually. The slave-side
eviction is encapsulated in code which is not coupled with the rest of
the Redis core, if not for the hook to track the keys.

TODO

I'm doing the first smoke tests to see if the feature works as expected:
so far so good. Unit tests should be added before merging into the
4.0 branch.
