Clean up the cache sub-directory in a background thread

[unexge]

Mountpoint tries to clean up the cache sub-directory during startup and exit in a blocking manner. This causes a cold start on Mountpoint if the cache sub-directory is not cleaned up in exit (e.g., if it's forcefully terminated) and if the cache sub-directory contains lots of files.

This change updates the clean up logic to do a rename first in the current thread to ensure Mountpoint starts with a fresh cache folder, and then spawns a detached background thread to remove the old cache directory. The cache sub-directory is still cleaned up in a blocking manner in the current thread during exit.

Does this change impact existing behavior?

The cache will now be cleaned up in a background thread, this change may:

• Increase disk space usage temporarily during cleanup, as the new cache begins populating while the old cache still exists for a period of time
• Prevent cache cleanup errors from being propagated, since Mountpoint startup is no longer blocked by the cache cleanup process. Errors will now be just logged instead of propagated
  • In the worst-case scenario, this could cause Mountpoint to leave orphaned cache folders

Does this change need a changelog entry? Does it require a version change?

Probably requires a changelog entry.

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