Better support for pub/sub.

Make the SUBSCRIBE/UNSUBSCRIBE commands not expect a reply in order to
avoid race conditions with incoming messages. Prevent similar race
conditions when returning a leased connection.
Fix double-return race.

A connection could be returned through the thread-dead-evt or
explicitly using disconnect.
Merge branch 'master' of https://github.com/m4burns/redis
update pub/sub tests
add more pubsub tests