pjsip_distributor.c: Consistently pick a serializer for messages.

Incoming messages that are not part of a dialog or a recognized response
to one of our requests need to be sent to a consistent serializer.  Under
load we may be queueing retransmissions before we can process the original
message.  We don't need to throw these messages onto random serializers
and cause reentrancy and message sequencing problems.

* Created a pool of pjsip/distributor serializers that get picked by
hashing the call-id and remote tag strings of the received messages.

* Made ast_sip_destroy_distributor() destroy items in the reverse order of
creation.

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: I2ce769389fc060d9f379977f559026fbcb632407