Bridge API: Set a cause code on a channel when it is ejected from a bridge.

The cause code needs to be passed from the disconnecting channel to the
bridge peers if the disconnecting channel dissolves the bridge.

* Made the call to an app_agent_pool agent disconnect with the busy cause
code if the agent does not ack the call in time or hangs up before acking
the call.

(closes issue ASTERISK-22042)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2772/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397472 65c4cc65-6c06-0410-ace0-fbb531ad65f3