MAX-157. Added support for hints in rwsplit router.

buffer.c: added memory release for hint of a GWBUF
hint.c: added bool hint_exists()
hint.h: added placeholder for hint type HINT_ROUTE_TO_ALL which doesn't have implementation yet.
filter/Makefile: fixed dependency issue
hintparser.c: added const char* token_get_keyword(), hint_parser:added NULL check, hint_next_token: fixed off-by-one bug
readwritesplit.h: added bitfield for hints' use, which includes route targets and flag for case where user hinted to route to named backend server.
readwritesplit.c: added function route_target_t get_route_target() for resolving route target based on 1) query type (from query_classifier) 2) transaction state (active/not) and 3) hints. Modified get_dcb, which is called in routeQuery to provide pointer to correct backend DCB. Now get_dcb also takes server unique name as a parameter if such a hint was found. for hints' use, which includes  enter the commit message for your changes.

query_classifier/query_classifier.cc

@@ -464,7 +464,7 @@ static skygw_query_type_t resolve_query_type(
                 type |= QUERY_TYPE_DISABLE_AUTOCOMMIT;  
                 type |= QUERY_TYPE_BEGIN_TRX;
         }
-        /**
+       /**
         * REVOKE ALL, ASSIGN_TO_KEYCACHE,
         * PRELOAD_KEYS, FLUSH, RESET, CREATE|ALTER|DROP SERVER
         */