WIP Optionally restore credit_flow between channel -> CQ processes

The credit_flow between publishing AMQP 0.9.1 channel and
(non-mirrored) classic queue processes was unintentionally removed in
4.0 together with anything else related to CQ mirroring.

By default we keep the 4.0 behaviour of no flow-control, but with the
new env `classic_queue_flow_control` it is possible to restore the 3.x
behaviour for non-mirored classic queues.

deps/rabbit/src/rabbit_channel.erl

@@ -159,7 +159,8 @@
              rejected,
              %% used by "one shot RPC" (amq.
              reply_consumer :: none | {rabbit_types:ctag(), binary(), binary()},
-             delivery_flow, %% Deprecated since removal of CMQ in 4.0
+             %% see rabbitmq-server#114
+             delivery_flow :: flow | noflow,
              interceptor_state,
              queue_states,
              tick_timer,
@@ -489,6 +490,10 @@ init([Channel, ReaderPid, WriterPid, ConnPid, ConnName, Protocol, User, VHost,
     ?LG_PROCESS_TYPE(channel),
     ?store_proc_name({ConnName, Channel}),
     ok = pg_local:join(rabbit_channels, self()),
+    Flow = case rabbit_misc:get_env(rabbit, classic_queue_flow_control, false) of
+             true   -> flow;
+             false  -> noflow
+           end,
     {ok, {Global0, Prefetch}} = application:get_env(rabbit, default_consumer_prefetch),
     Limiter0 = rabbit_limiter:new(LimiterPid),
     Global = Global0 andalso is_global_qos_permitted(),
@@ -537,6 +542,7 @@ init([Channel, ReaderPid, WriterPid, ConnPid, ConnName, Protocol, User, VHost,
                 rejected                = [],
                 confirmed               = [],
                 reply_consumer          = none,
+                delivery_flow           = Flow,
                 interceptor_state       = undefined,
                 queue_states            = rabbit_queue_type:init()
                },