res_pjsip: Fix deadlock when sending out-of-dialog requests.

The struct send_request_wrapper has a pjsip lock associated with it that
is created non-recursive.  There is a code path for the struct
send_request_wrapper lock that will attempt to lock it recursively.  The
reporter's deadlock showed that the thread calling endpt_send_request()
deadlocked itself right after the wrapper object got created.

Out-of-dialog requests such as MESSAGE, qualify OPTIONS, and unsolicited
MWI NOTIFY messages can hit this deadlock.

* Replaced the struct send_request_wrapper pjsip lock with the mutex lock
that can come with an ao2 object since all of Asterisk's mutexes are
recursive.  Benefits include removal of code maintaining the pjsip
non-recursive lock since ao2 objects already know how to maintain their
own lock and the lock will show up in the CLI "core show locks" output.

ASTERISK-25435 #close
Reported by: Dmitriy Serov

Change-Id: I458e131dd1b9816f9e963f796c54136e9e84322d