app_bridgewait: Remove possibility of race condition between channels leaving/joining.

Bridges created by app_bridgewait previously had the "dissolve when empty" flag set.
This caused the bridge core to destroy them when the last channel had left. This
introduced a race condition where we may have a reference to the bridge but it is
not actually joinable when we try to join it. This flag has now been removed and the
bridge is guaranteed to be joinable at all times.

ASTERISK-23987 #close
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3836/
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Merged revisions 419538 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419539 65c4cc65-6c06-0410-ace0-fbb531ad65f3