MXS-1624 Add configuration parameter

With the global configuration parameter 'query_classifier_cache'
the query classification cache can be turned on. At the moment it
does not matter what value it has; its presence simply enables the
caching.

Eventually you will be able to specify how much memory the cache
is allowed to consume.

include/maxscale/config.h

@@ -257,6 +257,7 @@ typedef struct
                                                         * promoted from a passive to an active */
     char          qc_name[PATH_MAX];                   /**< The name of the query classifier to load */
     char*         qc_args;                             /**< Arguments for the query classifier */
+    QC_CACHE_PROPERTIES* qc_cache_properties;          /**< The query classifier cache properties. */
     qc_sql_mode_t qc_sql_mode;                         /**< The query classifier sql mode */
     char          admin_host[MAX_ADMIN_HOST_LEN];      /**< Admin interface host */
     uint16_t      admin_port;                          /**< Admin interface port */