rabbit_backing_queue: pass the whole message to discard callback

The previous behaviour was passing solely the message ID making
queue implementations such as, for example, the priority one hard
to fulfil.

Signed-off-by: Matteo Cafasso <noxdafox@gmail.com>
(cherry picked from commit 1f7a27c)

Author: noxdafox

deps/rabbit/src/rabbit_backing_queue.erl

@@ -105,7 +105,7 @@
 
 %% Called to inform the BQ about messages which have reached the
 %% queue, but are not going to be further passed to BQ.
--callback discard(rabbit_types:msg_id(), pid(), state()) -> state().
+-callback discard(rabbit_types:basic_message(), pid(), state()) -> state().
 
 %% Return ids of messages which have been confirmed since the last
 %% invocation of this function (or initialisation).