MXS-862: Create common MySQL library

The MySQLCommon library contains functions used by both the protocol and
authenticator modules. The contents of the modutil.c file could also be
moved to this file if the functions in that file are only used by modules
and not the core.

server/modules/include/mysql_client_server_protocol.h

@@ -380,4 +380,7 @@ bool gw_read_backend_handshake(DCB *dcb, GWBUF *buffer);
 /** Send the server handshake response packet to the backend server */
 mxs_auth_state_t gw_send_backend_auth(DCB *dcb);
 
+/** Write an OK packet to a DCB */
+int mxs_mysql_send_ok(DCB *dcb, int sequence, int affected_rows, const char* message);
+
 #endif /** _MYSQL_PROTOCOL_H */