res_pjsip_session: Use distributor serializer for incoming calls.

We must continue using the serializer that the original INVITE came in on
for the dialog.  There may be retransmissions already enqueued in the
original serializer that can result in reentrancy and message sequencing
problems.

Outgoing call legs create the pjsip/outsess/<endpoint> serializers for
their dialogs.

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: I24d7948749c582b8045d5389ba3f6588508adbbc