MXS-1066 Add query hint to route to last used server

Add new hint type and support for it in the readwritesplit router.

include/maxscale/hint.h

@@ -31,6 +31,7 @@ typedef enum
     HINT_ROUTE_TO_NAMED_SERVER,
     HINT_ROUTE_TO_UPTODATE_SERVER, /*< not supported by RWSplit and HintRouter */
     HINT_ROUTE_TO_ALL, /*< not supported by RWSplit, supported by HintRouter */
+    HINT_ROUTE_TO_LAST_USED,
     HINT_PARAMETER
 } HINT_TYPE;
 

include/maxscale/queryclassifier.hh

@@ -117,7 +117,8 @@ public:
         TARGET_SLAVE        = 0x02,
         TARGET_NAMED_SERVER = 0x04,
         TARGET_ALL          = 0x08,
-        TARGET_RLAG_MAX     = 0x10
+        TARGET_RLAG_MAX     = 0x10,
+        TARGET_LAST_USED    = 0x20
     };
 
     static bool target_is_master(uint32_t t)
@@ -145,6 +146,11 @@ public:
         return (t & TARGET_RLAG_MAX);
     }
 
+    static bool target_is_last_used(uint32_t t)
+    {
+        return (t & TARGET_LAST_USED);
+    }
+
     enum current_target_t
     {
         CURRENT_TARGET_UNDEFINED, /**< Current target has not been set. */