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.

server/core/gateway.cc

@@ -1975,7 +1975,7 @@ int main(int argc, char **argv)
         goto return_main;
     }
 
-    if (!qc_setup(NULL, cnf->qc_sql_mode, cnf->qc_name, cnf->qc_args))
+    if (!qc_setup(cnf->qc_cache_properties, cnf->qc_sql_mode, cnf->qc_name, cnf->qc_args))
     {
         const char* logerr = "Failed to initialise query classifier library.";
         print_log_n_stderr(true, true, logerr, logerr, eno);