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.

server/modules/include/readwritesplit.h

@@ -67,10 +67,22 @@ typedef enum backend_type_t {
         BE_UNDEFINED=-1, 
         BE_MASTER, 
         BE_JOINED = BE_MASTER,
-        BE_SLAVE, 
+        BE_SLAVE,
         BE_COUNT
 } backend_type_t;
 
+typedef enum {
+        TARGET_MASTER       = 0x01,
+        TARGET_SLAVE        = 0x02,
+        TARGET_NAMED_SERVER = 0x04,
+        TARGET_ALL          = 0x08
+} route_target_t;
+
+#define TARGET_IS_MASTER(t)       (t & TARGET_MASTER)
+#define TARGET_IS_SLAVE(t)        (t & TARGET_SLAVE)
+#define TARGET_IS_NAMED_SERVER(t) (t & TARGET_NAMED_SERVER)
+#define TARGET_IS_ALL(t)          (t & TARGET_ALL)
+
 typedef struct rses_property_st rses_property_t;
 typedef struct router_client_session ROUTER_CLIENT_SES;