Add a `flush` handler to amqp_channel

rabbit_channel may use amqp_channel as the writer.
When terminating, rabbit_channel sends a `flush` message
to its writer. If amqp_channel is in use, that led to
a `function_clause` crash.

(cherry picked from commit 0ce6ad0f0f)

deps/amqp_client/src/amqp_channel.erl

@@ -384,6 +384,10 @@ init([Driver, Connection, ChannelNumber, Consumer, Identity]) ->
 handle_call(open, From, State) ->
     {noreply, rpc_top_half(#'channel.open'{}, none, From, none, noflow, State)};
 %% @private
+handle_call(flush, _From, State) ->
+    flush_writer(State),
+    {noreply, State};
+%% @private
 handle_call({close, Code, Text}, From, State) ->
     handle_close(Code, Text, From, State);
 %% @private