bridges/bridge_native_rtp: Reconfigure bridge on removal of framehook

This patch is a re-do of r414122.

When r414122 was merged, a major problem with it was uncovered. UNBRIDGE soft
hangup flags have a catastrophic effect on the pbx core if they leak out from
the bridge layer: the channel gets hung up. With the number of threads
involved in a blind transfer, and with the initial patch, it was likely that
this would occur. This caused a large number of test failures

This patch is nearly identical with the one proposed in r414122, save for the
following changes:
 - We explicitly clear the UNBRIDGE flag when setting an after goto on a
   channel in a bridge
 - Defensively, if we encounter an UNBRIDGE flag in the pbx core, we handle it

https://reviewboard.asterisk.org/r/3585/
........

Merged revisions 415443 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@415444 65c4cc65-6c06-0410-ace0-fbb531ad65f3