app_queue: Crash when transferring

During some transfer scenarios involving queues Asterisk would sometimes
crash when trying to obtain a channel snapshot (could happen on caller or
member channels). This occurred because the underlying channel had already
disappeared when trying to obtain the latest snapshot.

This patch adds a reference to both the member and caller channels that
extends to the lifetime of the queue'd call, thus making sure the channels
will always exist when retrieving the latest snapshots.

ASTERISK-25185 #close
Reported by: Etienne Lessard

Change-Id: Ic397fa68fb4ff35fbc378e745da9246a7b552128