MXS-1866: Add support for COM_STMT_BULK_EXECUTE

Readwritesplit now detects the COM_STMT_BULK_EXECUTE command and handles
it correctly.

include/maxscale/protocol/mysql.h

@@ -293,6 +293,7 @@ typedef enum
     MXS_COM_STMT_RESET = 26,
     MXS_COM_SET_OPTION = 27,
     MXS_COM_STMT_FETCH = 28,
+    MXS_COM_STMT_BULK_EXECUTE = 0xfa,
     MXS_COM_DAEMON,
     MXS_COM_END
 } mxs_mysql_cmd_t;