Refactor send_fun in protocol and other connection module

Prior to this change, in the send function, the packet is forced to
use emqx:serialize to serialize packet, it is a wrong design because
other plugins which need to transform the mqtt packet to other packets
can not use their own serialize function to serialize packet.

This change solve the problem issued above.

src/emqx_connection.erl

@@ -165,7 +165,8 @@ init_limiter({Rate, Burst}) ->
     esockd_rate_limit:new(Rate, Burst).
 
 send_fun(Transport, Socket, Peername) ->
-    fun(Data) ->
+    fun(Serialize, Packet, Options) ->
+        Data = Serialize(Packet, Options),
         try Transport:async_send(Socket, Data) of
             ok ->
                 ?LOG(debug, "SEND ~p", [iolist_to_binary(Data)], #state{peername = Peername}),
@@ -408,4 +409,3 @@ maybe_gc(#state{}, {publish, _PacketId, #message{payload = Payload}}) ->
     ok = emqx_gc:inc(1, Oct);
 maybe_gc(_, _) ->
     ok.
-

src/emqx_frame.erl

@@ -634,4 +634,3 @@ fixqos(?PUBREL, 0)      -> 1;
 fixqos(?SUBSCRIBE, 0)   -> 1;
 fixqos(?UNSUBSCRIBE, 0) -> 1;
 fixqos(_Type, QoS)      -> QoS.
-

src/emqx_protocol.erl

@@ -555,7 +555,7 @@ deliver({disconnect, _ReasonCode}, PState) ->
 -spec(send(emqx_mqtt_types:packet(), state()) -> {ok, state()} | {error, term()}).
 send(Packet = ?PACKET(Type), PState = #pstate{proto_ver = Ver, sendfun = SendFun}) ->
     trace(send, Packet, PState),
-    case SendFun(emqx_frame:serialize(Packet, #{version => Ver})) of
+    case SendFun(fun emqx_frame:serialize/2, Packet, #{version => Ver}) of
         ok ->
             emqx_metrics:sent(Packet),
             {ok, inc_stats(send, Type, PState)};

src/emqx_ws_connection.erl

@@ -144,7 +144,8 @@ websocket_init(#state{request = Req, options = Options}) ->
                 idle_timeout = IdleTimout}}.
 
 send_fun(WsPid) ->
-    fun(Data) ->
+    fun(Serialize, Packet, Options) ->
+        Data = Serialize(Packet, Options),
         BinSize = iolist_size(Data),
         emqx_metrics:inc('bytes/sent', BinSize),
         put(send_oct, get(send_oct) + BinSize),
@@ -299,4 +300,3 @@ stop(Error, State) ->
 
 wsock_stats() ->
     [{Key, get(Key)} || Key <- ?SOCK_STATS].
-