bridge: During a smart bridge operation provide a more complete bridge to the old technology.

When a smart bridge operation occurs and a bridge transitions from one
technology to another the old technology is provided the channels formerly
in it and told that they are leaving. Unfortunately the bridge provided
along with them is incomplete. The bridge, despite there being channels in it,
contains none. This forces technology implementations to have additional
logic when channels are leaving or to store their own duplicated
state.

This change makes the bridge more complete so it contains the expected
channels. Now that the bridge is complete special logic within
bridge_native_rtp is no longer needed and has been removed.

Review: https://reviewboard.asterisk.org/r/4057/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@425242 f38db490-d61c-443f-a65b-d21fe96a405b