pjsip_options: Fix continued qualifies after endpoint/aor deletion

pjsip_options: Fix continued qualifies after endpoint/aor deletion

If you remove an endpoint/aor from pjsip.conf then do a core reload,
qualifies will continue even though the object are gone.  This happens
because nothing clears out the qualify tasks.

This patch unschedules all existing qualify tasks before scheduling
new ones on reload.

Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/4290/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430064 65c4cc65-6c06-0410-ace0-fbb531ad65f3