This commit adds a scheduler API call, ast_sched_replace that can be used

in place of a very common construct.  I also used it in a number of places
in chan_sip.

  if (id > -1)
     ast_sched_del(sched, id);
  id = ast_sched_add(sched, ...);

changes to:

  ast_sched_replace(id, sched, ...);


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@79861 65c4cc65-6c06-0410-ace0-fbb531ad65f3