Bridge DTMF hooks: Made audio pass from the bridge while waiting for more matching digits.

* Made collecting DTMF digits for the DTMF feature hooks pass frames from
the bridge.

* Made collecting DTMF digits possible by other bridge hooks if there is a
need.

ASTERISK-24447 #close
Reported by: Richard Mudgett

Review: https://reviewboard.asterisk.org/r/4123/
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Merged revisions 427493 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 427494 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@427495 65c4cc65-6c06-0410-ace0-fbb531ad65f3