RFC6665 4.2.2: ... after a failed State NOTIFY transaction remove the subscription

The problem is that the State Notify requests rely on the 200OK reponse for pacing control
and to not confuse the notify susbsystem.
The issue is, the pendinginvite isn't cleared if a response isn't received,
thus further notify's are never sent.

The solution, follow RFC 6665 4.2.2's 'SHOULD' and remove the subscription after failure.
  
(closes issue ASTERISK-21677)

Reported by: Dan Martens
Tested by: Dan Martens, David Brillert, alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2475/
........

Merged revisions 387875 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@387880 65c4cc65-6c06-0410-ace0-fbb531ad65f3