Side loading compressed layer files into the cacheDir

### What happened in your environment?

is side loading compressed layer files into the cacheDir supported?

I am looking into fetching the layer files out of band and saving them in the cacheDir as sha256:. Does overlaybd pickup the new files added externally to the directory after it has started? Or does it hydrate once and maintains the inventory internally until a restart?

### What did you expect to happen?

I still see a few downloads happening - the logging wasn't verbose and didn't easily tell me which digest it was referring to - i will turn on debug logging and try again - but wanted to check if the above scenario is supposed to work.

### How can we reproduce it?

copy the compressed layer files to the cacheDir after overlaybd has started - then start the container - expectation is overlaybd doesn't issue any blob download requests.

### What is the version of your Overlaybd?

NA

### What is your OS environment?

Ubuntu 18

### Are you willing to submit PRs to fix it?

- [ ] Yes, I am willing to fix it.