Free cluster err on shutdown
Minor formatting changes
Allow WATCH even when the cluster is resharding.

We can actully execute multi commands like WATCH/MSET/MGET while
resharding, given that we'll just get bounced around until all
the keys are in the correct place.
MGET

Initial commit of logic for the MGET command, which will distribute
the commands across nodes in the cluster, based on how the slots hash.

The response for MGET in cluster differs from Redis proper in a couple
of ways.

1)  Except for a critical failure (e.g. being unable to write the
    command *anywhere*, or internal PHP HashTable malfunctions,
    the client will always get an array back, even on within the
    processing.

2)  This array will correspond 1-1 with the keys requested, so that
    the client can use the keys passed in array as a sentinel for
    the responses.

3)  When NO key is found, the resulting value will be NULL, whereas
    in the event of an error it will be FALSE.