Fix for a possible bug related to ZINTER/UNIONSTORE called with the same source set more than one time.

removed assert causing an illegal memory access. This was responsible of crashes during BLPOP and other list blocking operations.

RDB version signature bumped to 2, now that direct saving of specially encoded types is implemented 2.2 instances are no longer able to read rdb files produced by 2.4 or unstable.

Introduced a safe iterator interface that can be used to iterate while accessing the dictionary at the same time. Now the default interface is consireded unsafe and should be used only with dictNext()

DEBUG DIGEST additional lookup needed for VM removed from unstable branch that does not have VM at all

when Redis fails accepting a new connection reports the error at WARNING and not VERBOSE error level. Thanks to offby1 for proposing this in the Redis mailing list. #backport-candidate

Fixed semantics of CLUSTER SETSLOT, SELECT now only denied in cluster mode if selected DB is not 0 so that MIGRATE still works well.