caddytls: Allow disabling storage cleaning, avoids writing two files

[francislavoie]

Some users want to run Caddy in a totally read-only mode, but currently storage cleaning implicitly creates two files instance.uuid and last_clean.json which are undesirable in that situation. We can simply skip starting the storage cleaning goroutine via a config option.

I noticed storage_check off wasn't configurable in the Caddyfile, so I added that too, while I was at it.

[mholt]

Thanks for working on this! A couple questions/thoughts:

• What is the harm of trying to write these two files?
• Maybe we should just have a read_only mode in the global options (if it is actually needed). I don't know if I love this granularity since you need to turn several knobs for one thing.

[francislavoie]

Forgot to link the related forum thread https://caddy.community/t/disabling-file-storage/25772, those 2 files get written which is undesirable in a setup with the goal of being read-only.

The way I see it, doesn't hurt to have this option, solves an edgecase for someone. Very simple implementation.

Maybe we should just have a read_only mode in the global options (if it is actually needed). I don't know if I love this granularity since you need to turn several knobs for one thing.

I dunno, an option that does too much is not better.