Merge changes from team/russell/sla_updates.

This batch of changes to the SLA code does a few different things.

* I made the SLA code event driven instead of having to act in a lot of busy
  loops while dialing things to wait for state changes.  This makes the code
  more efficient and readable at the same time.

* I have implemented a couple of new features.  The first is inbound trunk
  ringing timeouts.  This is an option that defines how long to let an incoming
  call on a trunk to ring.

* I have also implemented ring timeouts for stations.  They may be specified
  for the entire station, meaning it is how long to let the station ring before
  giving up.  You can also specify a ring timeout for a specific trunk on a
  station.  So, you can say that you only want a specific station to ring 5
  seconds if it is line1 ringing, but otherwise, there is no timeout.


git-svn-id: http://svn.digium.com/svn/asterisk/branches/1.4@56277 f38db490-d61c-443f-a65b-d21fe96a405b

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