The member can hangup the call while we are playing the outbound announce audio to the agent, this
patch detects this and properly hangup the agent channel to avoid keeping it up with
just a silence call since there's no active bridge.

The agent-bridge-end event now has CC-Agent-Bridged: true if the bridge successfully happens,
variable cc_agent_bridged was added so we can track this in the agent and member CDRs.

If you want to play an audio to the agent to indicate the member hangup the call before the bridge
set the variable cc_bridge_failed_outbound_announce, for example:
<action application="set" data="cc_bridge_failed_outbound_announce=tone_stream://%(250,250,425);loops=3"/>
or
<action application="set" data="cc_bridge_failed_outbound_announce=your-caller-has-gone-away-audio"/>
prior to calling callcenter application.

These variables are documented at https://freeswitch.org/confluence/display/FREESWITCH/mod_callcenter