Cluster: getKeysFromCommand() and related: top-comments added.
Cluster: evalGetKey() added for EVAL/EVALSHA.

Previously we used zunionInterGetKeys(), however after this function was
fixed to account for the destination key (not needed when the API was
designed for "diskstore") the two set of commands can no longer be served
by an unique keys-extraction function.
Cluster: getKeysFromCommand() top comment improved.