Formatting variables and then passing them into the logger functions leads
to performance issues. i.e.
```erlang
logger:debug("RECV ~s", [emqx_packet:format(Packet)])
```
Above message will only be printed when the current log level set to
`debug`, but the function emqx_packet:format/1 will always be evaluated no
matter what the current log level is.
OTP 21 provides a special meta-data named `report_cb`, which can be used
for lazy evaluation. The fun is only evaluated if the primary/handler log level
check passes, and is therefore recommended if it is expensive to generate
the message.
Prior to this change, there are ok, {binary, _Data}. {datagram, _Peer,
_Dara} case clauses, and the {binary, _Data} and {datagram, _Peer,
_Data} are unnecessary cases
This change delete these two cases and add ok in the end of funtion in
send_fun of emqx_ws_connection.
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.
terry-xiaoyu