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

main/threadpool.c

@@ -1259,13 +1259,17 @@ static struct serializer *serializer_create(struct ast_threadpool *pool,
 	return ser;
 }
 
+AST_THREADSTORAGE_RAW(current_serializer);
+
 static int execute_tasks(void *data)
 {
 	struct ast_taskprocessor *tps = data;
 
+	ast_threadstorage_set_ptr(&current_serializer, tps);
 	while (ast_taskprocessor_execute(tps)) {
 		/* No-op */
 	}
+	ast_threadstorage_set_ptr(&current_serializer, NULL);
 
 	ast_taskprocessor_unreference(tps);
 	return 0;
@@ -1305,6 +1309,11 @@ static struct ast_taskprocessor_listener_callbacks serializer_tps_listener_callb
 	.shutdown = serializer_shutdown,
 };
 
+struct ast_taskprocessor *ast_threadpool_serializer_get_current(void)
+{
+	return ast_threadstorage_get_ptr(&current_serializer);
+}
+
 struct ast_taskprocessor *ast_threadpool_serializer_group(const char *name,
 	struct ast_threadpool *pool, struct ast_serializer_shutdown_group *shutdown_group)
 {