MXS-1196: Do not parse EXPLAIN statements

EXPLAIN statements are no longer parsed completely as doing so makes
it hard to modify the grammar for the needs or Oracle SQL.

Consequently, for an EXPLAIN statement you now bascially only get the
type and the operation (the newly added QUERY_OP_EXPLAIN and QUERY_OP_SHOW).
The other information is not interesting and is related to
information_schema and similar tables.

server/core/query_classifier.cc

@@ -494,42 +494,48 @@ const char* qc_op_to_string(qc_query_op_t op)
     case QUERY_OP_UNDEFINED:
         return "QUERY_OP_UNDEFINED";
 
-    case QUERY_OP_SELECT:
-        return "QUERY_OP_SELECT";
-
-    case QUERY_OP_UPDATE:
-        return "QUERY_OP_UPDATE";
-
-    case QUERY_OP_INSERT:
-        return "QUERY_OP_INSERT";
-
-    case QUERY_OP_DELETE:
-        return "QUERY_OP_DELETE";
-
-    case QUERY_OP_TRUNCATE:
-        return "QUERY_OP_TRUNCATE";
-
     case QUERY_OP_ALTER:
         return "QUERY_OP_ALTER";
 
-    case QUERY_OP_CREATE:
-        return "QUERY_OP_CREATE";
-
-    case QUERY_OP_DROP:
-        return "QUERY_OP_DROP";
-
     case QUERY_OP_CHANGE_DB:
         return "QUERY_OP_CHANGE_DB";
 
-    case QUERY_OP_LOAD:
-        return "QUERY_OP_LOAD";
+    case QUERY_OP_CREATE:
+        return "QUERY_OP_CREATE";
+
+    case QUERY_OP_DELETE:
+        return "QUERY_OP_DELETE";
+
+    case QUERY_OP_DROP:
+        return "QUERY_OP_DROP";
+
+    case QUERY_OP_EXPLAIN:
+        return "QUERY_OP_EXPLAIN";
 
     case QUERY_OP_GRANT:
         return "QUERY_OP_GRANT";
 
+    case QUERY_OP_INSERT:
+        return "QUERY_OP_INSERT";
+
+    case QUERY_OP_LOAD:
+        return "QUERY_OP_LOAD";
+
     case QUERY_OP_REVOKE:
         return "QUERY_OP_REVOKE";
 
+    case QUERY_OP_SELECT:
+        return "QUERY_OP_SELECT";
+
+    case QUERY_OP_SHOW:
+        return "QUERY_OP_SHOW";
+
+    case QUERY_OP_TRUNCATE:
+        return "QUERY_OP_TRUNCATE";
+
+    case QUERY_OP_UPDATE:
+        return "QUERY_OP_UPDATE";
+
     default:
         return "UNKNOWN_QUERY_OP";
     }