2 updates to storing configs on disk

1. When an existing config is updated, remove the outdated config from
disk to stop accumulating configs.
2. Check disk to remove configs to make sure we delete lingering files
that does not live on memory.

Test: statsd, statsd_test, manual
Change-Id: Iedce4b6eb99a3d36bb5e1d1ccc0d88c84859e8f5