res_parking: Replace Parker snapshots with ParkerDialString

This process also involved a large amount of rework regarding how to redial
the Parker when a channel leaves a parking lot due to timeout. An attended
transfer channel variable has been added to attended transfers to extensions
that will eventually park (but haven't at the time of transfer) as well.
This resolves one of the two BUGBUG comments remaining in res_parking.

(issues ASTERISK-21877)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2638/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393704 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE.txt

@@ -75,6 +75,22 @@ AMI:
    - Channels no longer swap Uniqueid's as a result of the masquerade.
    - Instead of a shell game of renames, there's now a single rename, appending
      <ZOMBIE> to the name of the original channel.
+ - The AMI events 'ParkedCall', 'ParkedCallTimeOut', 'ParkedCallGiveUp', and
+   'UnParkedCall' have changed significantly in the new res_parking module.
+   - The 'Channel' and 'From' headers are gone. For the channel that was parked
+     or is coming out of parking, a 'Parkee' channel snapshot is issued and it
+     has a number of fields associated with it. The old 'Channel' header relayed
+     the same data as the new 'ParkeeChannel' header.
+   - The 'From' field was ambiguous and changed meaning depending on the event.
+     for most of these, it was the name of the channel that parked the call
+     (the 'Parker'). There is no longer a header that provides this channel name,
+     however the 'ParkerDialString' will contain a dialstring to redial the
+     device that parked the call.
+   - On UnParkedCall events, the 'From' header would instead represent the
+     channel responsible for retrieving the parkee. It receives a channel
+     snapshot labeled 'Retriever'. The 'from' field is is replaced with
+     'RetrieverChannel'.
+   - Lastly, the 'Exten' field has been replaced with 'ParkingSpace'.
 
 CEL:
  - The Uniqueid field for a channel is now a stable identifier, and will not