MXS-1367: Retry interrupted queries

The new `query_retries` parameter controls how many times an interrupted
query is retried. This retrying of interrupted queries will reduce the
rate of false positives that MaxScale monitors detect.

server/core/maxscale/config.h

@@ -25,6 +25,7 @@ MXS_BEGIN_DECLS
 #define DEFAULT_NBPOLLS         3       /**< Default number of non block polls before we block */
 #define DEFAULT_POLLSLEEP       1000    /**< Default poll wait time (milliseconds) */
 #define DEFAULT_NTHREADS        1       /**< Default number of polling threads */
+#define DEFAULT_QUERY_RETRIES   0       /**< Number of retries for interrupted queries */
 
 /**
  * @brief Generate default module parameters