notifyKeyspaceEvent(): release channel names using the right pointers.
Fix decrRefCount() prototype from void to robj pointer.

decrRefCount used to get its argument as a void* pointer in order to be
used as destructor where a 'void free_object(void*)' prototype is
expected. However this made simpler to introduce bugs by freeing the
wrong pointer. This commit fixes the argument type and introduces a new
wrapper called decrRefCountVoid() that can be used when the void*
argument is needed.
Keyspace events added for more commands.
Enable keyspace events notification when testing.
Merge remote-tracking branch 'upstream/notifications' into notifications

Conflicts:
	src/notify.c