Stasis: Add information to blind transfer event

When a blind transfer occurs that is forced to create a local channel
pair to satisfy the transfer request, information about the local
channel pair is not published. This adds a field to describe that
channel to the blind transfer message struct so that this information
is conveyed properly to consumers of the blind transfer message.

This also fixes a bug in which Stasis() was unable to properly identify
the channel that was replacing an existing Stasis-controlled channel
due to a blind transfer.

Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/3921/
........

Merged revisions 421537 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@421538 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/events.json

@@ -285,6 +285,11 @@
 					"required": true,
 					"type": "Channel"
 				},
+				"replace_channel": {
+					"description": "The channel that is replacing transferer when the transferee(s) can not be transferred directly",
+					"required": false,
+					"type": "Channel"
+				},
 				"transferee": {
 					"description": "The channel that is being transferred",
 					"required": false,