chan_iax2: Prevent deadlock between hangup and sending lagrq/ping

chan_iax2: Prevent deadlock between hangup and sending lagrq/ping

channels/chan_iax.c: Prevent the deadlock between iax2_hangup and send_lagrq/
send_ping. This deadlock happens because the scheduled task send_lagrq(or
send_ping) starts execution after the call hangup procedure starts but before
it deletes the tasks in the scheduler.

The solution is to delete scheduled lagrq (and ping) task asynchronously
(i.e. schedule AST_SCHED_DEL for these tasks); By this, AST_SCHED_DEL will
be called in a new context (doesn't have callno locked).

This commit also cleans up the procedure of sending LAGRQ and PING.

main/sched.c: Do not assert when deleting non existant entry from scheduler.
This assert seems to be the reason for a lot of awkward code to avoid it.

ASTERISK-24983 #close
Reported by: Y Ateya

Change-Id: I03bec1fc8faacb89630269e935fa667c6d6c080c