Unlink session when exit message has been forwarded to session (#2703)

* Unlink session when exit message has been forwarded to session

* Readjust possition of emqx_protocol:session

src/emqx_ws_channel.erl

@@ -328,6 +328,7 @@ terminate_session(Reason, ProtoState) ->
         undefined ->
             ok;
         SessionPid ->
+            unlink(SessionPid),
             SessionPid ! {'EXIT', self(), Reason}
     end.