res_pjsip: Need to use the same serializer for a pjproject SIP transaction.

All send/receive processing for a SIP transaction needs to be done under
the same threadpool serializer to prevent reentrancy problems inside
pjproject and res_pjsip.

* Add threadpool API call to get the current serializer associated with
the worker thread.

* Pick a serializer from a pool of default serializers if the caller of
res_pjsip.c:ast_sip_push_task() does not provide one.

This is a simple way to ensure that all outgoing SIP request messages are
processed under a serializer.  Otherwise, any place where a pushed task is
done that would result in an outgoing out-of-dialog request would need to
be modified to supply a serializer.  Serializers from the default
serializer pool are picked in a round robin sequence for simplicity.

A side effect is that the default serializer pool will limit the growth of
the thread pool from random tasks.  This is not necessarily a bad thing.

* Made pjsip_distributor.c save the thread's serializer name on the
outgoing request tdata struct so the response can be processed under the
same serializer.

This is a cherry-pick from master.

**** ASTERISK-25115 Change-Id: Iea71c16ce1132017b5791635e198b8c27973f40a

NOTE: session_inv_on_state_changed() is disassociating the dialog from the
session when the invite dialog becomes PJSIP_INV_STATE_DISCONNECTED.
Unfortunately this is a tad too soon because our BYE request transaction
has not completed yet.

ASTERISK-25183 #close
Reported by: Matt Jordan

Change-Id: I8bad0ae1daf18d75b8c9e55874244b7962df2d0a