SS7 Native Bridge

Native bridge is enabled on 2 conditions:

* The SIP header FreeTDM-TransUUID is set in the originating leg and matches a freetdm channel
* The variable freetdm_native_sigbridge is true and the originating leg is also a freetdm channel

Some coding rules apply to this feature:

- Each channel is responsible for clearning its own peer_data and event queue
  at the end of the call (when moving to DOWN state)

- Each channel dequeues messages only from its own queue and enqueues messages
  in the peer's queue, with the only exception being messages received before
  the bridge is stablished (IAM for sure and possible SAM messages) because
  if the bridge is not yet stablished the messages must be queued by the channel
  in its own queue temporarily until the bridge is ready

- When the bridge is ready it is the responsibility of the incoming channel to
  move the messages that stored temporarily in its own queue to the bridged peer queue

- During hangup, each channel is responsible for moving itself to DOWN. The procedure
  however differs slightly depending on the hangup conditions

  If the user requests hangup (ie, FreeSWITCH) the request will be noted by setting the 
  FTDM_CHANNEL_USER_HANGUP flag but will not be processed yet because call control is
  driven only by the link messages (so no hangup from ESL or command line allowed)

  When REL message comes, the channel receiving it must move to TERMINATING state and:
	    
	- If the user has not hangup yet (FTDM_CHANNEL_USER_HANGUP flag not set) then
	  notify the user via SIGEVENT_STOP and wait for the user to move to HANGUP
	  state by calling ftdm_channel_call_hangup() before sending RLC 

	- If the user did hangup already (FTDM_CHANNEL_USER_HANGUP flag is set) then
	  skip user notification and move to HANGUP state directly where the RLC message
	  will be sent

- On HANGUP state the RLC is sent and the channel is moved to DOWN, final state
  The peer channel will forward the REL message and wait for RLC from the network, when
  RLC is received the channel can move straight to DOWN itself because the peer channel
  is completing its own shutdown procedure when it received the REL message

