res_pjsip_transport_management.c: Misc cleanups to survive shutdown.

* In unload_module(), reordered destroying things to minimize the window
that the global transports container could be used by other threads on
shutdown.  When shutting down you need to stop things in the opposite
order of creation.

* Put the global transports container into an AO2_GLOBAL_OBJ_STATIC to
eliminate the crash potential by other threads using the container on
shutdown.

* Made struct monitored_transport.sip_received not use
ast_atomic_fetchadd_int() since it is used as a boolean value that is only
set TRUE.  It was previously incremented for every received SIP message
and could theoretically overflow.

* In monitored_transport_state_callback(), allocated the monitored
transport object without a lock since the lock was unused.

* In keepalive_global_loaded(), removed releasing the transports container
if the keepalive_thread could not be started.  I set it up to be tried
again if the user reloads the configuration.

Change-Id: I8d12d16ef564290fa6d25a32334bb5ce8fdf87ff