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/maxinfo/maxinfo.h

@@ -100,13 +100,6 @@ typedef struct maxinfo_tree
 
 
 #define MYSQL_COMMAND(buf)  (*((uint8_t *)GWBUF_DATA(buf) + 4))
-/**
- * MySQL protocol OpCodes needed for replication
- */
-#define COM_QUIT                0x01
-#define COM_QUERY               0x03
-#define COM_STATISTICS              0x09
-#define COM_PING                0x0e
 
 /**
  * Token values for the tokeniser used by the parser for maxinfo