pjsip_scheduler.c: Fix ao2 usage errors.

* Removed several invalid uses of OBJ_NOLOCK.  These uses resulted in the
'tasks' container being accessed without a lock in a multi-threaded
environment.  A recipe for crashes.

* Removed needlessly obtaining schtd object references.  If the caller
providing you a pointer to an object doesn't have a valid reference then
you cannot safely get one from it.

* Getting a ref to 'tasks' when you aren't copying the pointer into
another location is useless.  The 'tasks' container pointer is global.

* Removed many unnecessary uses of RAII_VAR.

* Make ast_sip_schedule_task() name parameter const.

ASTERISK_26806

Change-Id: I5c62488e651314e2a1dbc01f5b078a15512d73db