Removed redundancy from error and message logs.

server.c: Added new member to SERVER->master_err_is_logged. It is used if server loses master status. It causes error log print in readwritesplit router's eror handling. Initial value is false and it is set always to false when server's status is set to master.
Added message log printing to mysql monitor, if master status changes to something else. It is not warning or error but only information which probably interests the user.

readwritesplit.c:Muted warnings and error printings in cases if slaves are not found if it is allowed to have a master only.
readwritesplit.c:Corrected error log printing in case where master lost its status. REdundant prints are removed.

server/core/service.c

@@ -356,7 +356,7 @@ int		listeners = 0;
 			"%s: Failed to create router instance for service. Service not started.",
 				service->name)));
 		service->state = SERVICE_STATE_FAILED;
-		return NULL;
+		return 0;
 	}
 
 	port = service->ports;