Changed background_rollback to return true/false depending on if

background rollback did happen.
Background rollback should be skipped if the aborting happens due to KILL command issued by user.
Some KILL signals, like KILL CONECTION, wake up the victim too early so that background rollback
could happen in parallel with the victim waking up and continuing execution.

test/mock_server_state.hpp

@@ -183,11 +183,12 @@ namespace wsrep
             return start_sst_action();
         }
 
-        void
+        bool
         background_rollback(wsrep::client_state& client_state) WSREP_OVERRIDE
         {
             client_state.before_rollback();
             client_state.after_rollback();
+            return false;
         }
 
         int wait_committing_transactions(int) WSREP_OVERRIDE { return 0; }