Introduce internal protocol command enum

The enums exposed by the connector are not intended to be used by the
users of the library. The fact that the protocol, and other, modules used
it was in violation of how the library is intended to be used.

Adding an internal mapping into MaxScale also removes some of the
dependencies that the core has on the connector.

server/modules/routing/readwritesplit/rwsplit_tmp_table_multi.cc

@@ -197,7 +197,7 @@ bool check_for_multi_stmt(GWBUF *buf, void *protocol, uint8_t packet_type)
     bool rval = false;
 
     if (proto->client_capabilities & GW_MYSQL_CAPABILITIES_MULTI_STATEMENTS &&
-        packet_type == MYSQL_COM_QUERY)
+        packet_type == MXS_COM_QUERY)
     {
         char *ptr, *data = (char*)GWBUF_DATA(buf) + 5;
         /** Payload size without command byte */