MXS-553: Provide full session to DCB mapping

By storing a link to the backend DCBs in the session object itself, we can
reach all related objects from the session. This removes the need to
iterate over all DCBs to find the set of related DCBs.

include/maxscale/session.h

@@ -29,12 +29,15 @@
 
 #ifdef __cplusplus
 #include <tr1/unordered_map>
+#include <tr1/unordered_set>
 #include <string>
 #include <deque>
 #include <vector>
 typedef std::deque<std::vector<uint8_t> > SessionStmtQueue;
+typedef std::tr1::unordered_set<DCB*> DCBSet;
 #else
 typedef void SessionStmtQueue;
+typedef void DCBSet;
 #endif
 
 MXS_BEGIN_DECLS
@@ -211,6 +214,7 @@ typedef struct session
         GWBUF* buffer;   /*< Buffer to deliver to up. */
     } response;                               /*< Shortcircuited response */
     SessionStmtQueue*      last_statements;   /*< The N last statements by the client */
+    DCBSet*                dcb_set;           /*< Set of associated backend DCBs */
     skygw_chk_t     ses_chk_tail;
 } MXS_SESSION;