In dcb.c:dcb_close DCB is removed either before or after the call dcb->func.close. Since mysql backend protocol sends COM_QUIT and thus, writes to backend DCB, it is kept in DCB_STATE_POLLING until the write is completed.

dcb.h: define ERRHAND temporarily since changes are still behind that macro
Defined two error handling actions in router.h: ERRACT_NEW_COMMECTION and ERRACT_REPLY_CLIENT.
Failed database is logged at expanding frequence to error and to message log due changes in mysql_mon.c. Added two new members in MONITOR_SERVERS: mon_err_count, and mon_prev_status so that each backend can be treated individually.

Error handling: if mysql_backend.c:dcb_read fails, router's handleError is called instead of closing session.
	If mysql_client.c:SESSION_ROUTE_QUERY fails router's handleError is called instead of sending error to client.

	readwritesplit.c:select_connect_backend_servers is modified so that in can be called during active router session. When called, it attempts to find one master and maximum number of configured slaves in correct state if necessary.
	When handleError needs to replace failed unit it now calls select_connect_backend_servers.

server/modules/monitor/mysqlmon.h

@@ -42,6 +42,8 @@
 typedef struct monitor_servers {
 	SERVER		*server;	/**< The server being monitored */
 	MYSQL		*con;		/**< The MySQL connection */
+	int             mon_err_count;
+        unsigned int    mon_prev_status;
 	struct monitor_servers
 			*next;		/**< The next server in the list */
 } MONITOR_SERVERS;