CQ: Fix entry missing from cache leading to crash on read

The issue comes from a mechanic that allows us to avoid writing
to disk when a message has already been consumed. It works fine
in normal circumstances, but fan-out makes things trickier.

When multiple queues write and read the same message, we could
get a crash. Let's say queues A and B both handle message Msg.

* Queue A asks store to write Msg
* Queue B asks store to write Msg
* Queue B asks store to delete Msg (message was immediately consumed)
* Store processes Msg write from queue A
  * Store writes Msg to current file
* Store processes Msg write from queue B
  * Store notices queue B doesn't need Msg anymore; doesn't write
  * Store clears Msg from the cache
* Queue A tries to read Msg
  * Msg is missing from the cache
  * Queue A tries to read from disk
  * Msg is in the current write file and may not be on disk yet
  * Crash

The problem is that the store clears Msg from the cache. We need
all messages written to the current file to remain in the cache
as we can't guarantee the data is on disk when comes the time
to read. That is, until we roll over to the next file.

The issue was that a match was wrong, instead of matching a single
location from the index, the code was matching against a list. The
error was present in the code for almost 13 years since commit
2ef30dc.

Author: Loïc Hoguin

deps/rabbit/src/rabbit_msg_store.erl

@@ -907,7 +907,7 @@ handle_cast({write, CRef, MsgRef, MsgId, Flow},
             %% the normal logic for that in write_message/4 and
             %% maybe_roll_to_new_file/2.
             case index_lookup(MsgId, State) of
-                [#msg_location { file = File }]
+                #msg_location { file = File }
                   when File == State #msstate.current_file ->
                     ok;
                 _ ->