Rclone mount cache and fragmentation

[Animosity022]

@spotter - fyi, check this out on read only files for the cache-mode full as I made an incorrect statement earlier:

Large object on rclone mount that exceeds local fs storage capacity

That's good to know as I was just testing it out and does seem to work ok for reading and it has some bumps when it runs out of space, but seems not too bad. I was testing with a 2G tmpfs to see what would happen and I could copy a large file with only a 2GB mount. Not sure I'd stream with something like that, but that's a great fix.