FS-9609 - [mod_callcenter] BLF for Queues, callcenter_track app, EXIT_WITH_KEY reason and xml_curl improvements

This change implements a few new features, there's no behavior change at all.

1 - BLF for queues

You can now use BLF for knowing when there are callers waiting on a queue,
the pattern to be subscribed is callcenter+queuename@default or @domain name.

2 - callcenter_track app

Now every agent has an external_calls_count column that will be read upon
selecting agent to offer new calls.
If you start an external call (inbound or outbound) and don't wan't
mod_callcenter to offer new calls for this agent just call this app
passing the agent name as argument:

<action application="callcenter_track" data="agent1"/>

This will increment the external calls count, and to make mod_callcenter look to this variable you need to set this queue parameter:

<param name="skip-agents-with-external-calls" value="true"/>

3 - EXIT_WITH_KEY reason

If the member press a key to exit the queue the cc_cancel_reason will be this now.

4 - xml_curl config loading improvements

Prior to this mod_callcenter query your web server a LOT of times,
this will make it only query once per queue, just watch for CC-Queue
request param and build your xml with everything related to that queue,
including tiers (tiers wasn't loaded before this).