app_agent_pool: Return to dialplan if the agent fails to ack the call.

Improvements to the agent pool functionality.

* AgentRequest no longer hangs up the caller if the agent fails to connect
with the caller.  It now continues in the dialplan.

* AgentRequest returns AGENT_STATUS set to NOT_CONNECTED if the agent
failed to connect with the call.  Most likely because the agent did not
acknowledge the call in time or got disconnected.

* The agent alerting play file configured by the agent.conf custom_beep
option can now be disabled by setting the option to an empty string.  The
agent is effectively alerted to a call presence when MOH stops.

* Fixed bridge reference leak when the agent connects with a caller.

ASTERISK-23499 #close
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3551/
........

Merged revisions 414747 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@414748 65c4cc65-6c06-0410-ace0-fbb531ad65f3