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/core/internal/session.h

@@ -51,6 +51,14 @@ const char *session_state(mxs_session_state_t);
  */
 void session_link_backend_dcb(MXS_SESSION *session, struct dcb *dcb);
 
+/**
+ * Unlink a session to a backend DCB.
+ *
+ * @param session  The session to unlink with the dcb
+ * @param dcb      The backend DCB to be unlinked
+ */
+void session_unlink_backend_dcb(MXS_SESSION *session, struct dcb *dcb);
+
 RESULTSET *sessionGetList(SESSIONLISTFILTER);
 
 void printAllSessions();