Utility command for cleaning up cache

[Michad]

To consider:

It's easy to neglect your cache if using an unbounded option such as S3. It would be nice to have a command that helps clean up a cache. At the very least have an option to delete all cache items for a specific layer. Nicer would be the ability to get a report of layers that exist in cache but not configuration.

Downside: this would require substantially more complexity in the cache interface. There's many edge cases that aren't trivial to handle well when you consider layers can be parameterized. Cleaning up is specific to the cache and some make it tricky to handle right, meanwhile dedicated tools for deleting data exists. As well currently there's a strong division of responsibility where tilegroxy writes but does not do any cleanup beyond setting a TTL where available - this blurs that and makes it expected that tilegroxy will handle the full cache lifecycle itself.

Probably need a decision record for this topic.