Create work around for scheduler leaks during shutdown.

Create work around for scheduler leaks during shutdown.

* Added ast_sched_clean_by_callback for cleanup of scheduled events
  that have not yet fired.
* Run all pending peercnt_remove_cb and replace_callno events in chan_iax2.
  Cleanup of replace_callno events is only run 11, since it no longer
  releases any references or allocations in 13+.

ASTERISK-24451 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4425/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@431916 65c4cc65-6c06-0410-ace0-fbb531ad65f3