Shard shared_SUITE

such that MQTT and WebMQTT tests of the shared_SUITE can run in parallel.
Before this commit, the shared_SUITE runs 14 minutes, after this commit
the shared_SUITE runs 4 minutes in GitHub actions.

Author: ansd

deps/rabbitmq_mqtt/BUILD.bazel

@@ -225,8 +225,8 @@ rabbitmq_integration_suite(
 
 rabbitmq_integration_suite(
    name = "shared_SUITE",
+   shard_count = 6,
    size = "large",
-   timeout = "eternal",
    runtime_deps = [
        "@emqtt//:erlang_app",
        "//deps/rabbitmq_management_agent:erlang_app",

deps/rabbitmq_mqtt/src/rabbit_mqtt_processor.erl

@@ -166,7 +166,7 @@ process_connect(
                        peer_port = PeerPort,
                        send_fun = SendFun,
                        exchange = rabbit_misc:r(VHost, exchange, rabbit_mqtt_util:env(exchange)),
-                       retainer_pid = rabbit_mqtt_retainer_sup:child_for_vhost(VHost),
+                       retainer_pid = rabbit_mqtt_retainer_sup:start_child_for_vhost(VHost),
                        vhost = VHost,
                        client_id = ClientId,
                        will_msg = make_will_msg(Packet)},