Reset persistent MySQL connections

When a persistent connection is taken from the pool, the state is reset
with a COM_CHANGE_USER on the next write. This allows reuse of persistent
connections without having to worry about the state of the MySQL session.

include/maxscale/dcb.h

@@ -276,6 +276,7 @@ typedef struct dcb
     bool            ssl_write_want_read;    /*< Flag */
     bool            ssl_write_want_write;    /*< Flag */
     int             dcb_port;       /**< port of target server */
+    bool            was_persistent;  /**< Whether this DCB was in the persistent pool */
     skygw_chk_t     dcb_chk_tail;
 } DCB;