Backup restore clear cache

[nichwall]

Fixes #3272

This PR purges the entire cache in /metadata/cache after restoring from a backup. The main reasoning for this is that backups may have different images or be done when migrating to a new server, so the cache should be cleared regardless.

I also updated the CacheManager to use ensureDir to be a little more readable.

[advplyr]

The reason ensureDir wasn't used there initially is because we used to set the UID/GID from the env variables but now that we aren't doing that ensureDir is better. Thanks!