Replication of scripts as EVALSHA: sha1 caching implemented.

This code is only responsible to take an LRU-evicted fixed length cache
of SHA1 that we are sure all the slaves received.

In this commit only the implementation is provided, but the Redis core
does not use it to actually send EVALSHA to slaves when possible.
Use the RSC to replicate EVALSHA unmodified.

This commit uses the Replication Script Cache in order to avoid
translating EVALSHA into EVAL whenever possible for both the AOF and
slaves.
Move Replication Script Cache initialization in safer place.

It should be called just one time at startup and not every time the Lua
scripting engine is re-initialized, otherwise memory is leaked.
SCRIPT FLUSH comment minor pedantic improvement.
Force propagation of SCRIPT LOAD to AOF.