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.

server/modules/filter/test/mock_session.cc

@@ -45,6 +45,8 @@ Session::Session(Client* pClient)
 Session::~Session()
 {
     delete variables;
+    delete last_statements;
+    delete dcb_set;
 }
 
 Client& Session::client() const