add 'flush' support

Add support to flush every data in cache to disk at maximal speed.
missed this, will fix (planned to do async api, but carbonlink is sync and it will be hard to make it async). about MAX_UPDATES_PER_SECOND, you missed prefix argument, which flush cache partially (for example, if you want flush cache for 'some_server.*'. just flush(prefix='some_server.')),
which cannot be done using MAX_UPDATES_PER_SECOND. And there is no sleep, so it will be flushed immediately.