res_pjsip_pubsub.c: Recreate subscriptions using distributor serializer.

* Resolves potential reentrancy problems if system restarted in the middle
of subscription message transactions.

* Fixes memory leak recreating persistent subscriptions when the
subscription resource tree could not be created.

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: I71e34d7ae8ed35a694f1030e820e2548c48697be