Monitor available disk space for local cache expiry
If casd is running without a quota or with a quota that is too large (larger than the sum of already used disk space and available disk space), it currently allows writes until completely running out of disk space, never triggering expiry.
I suggest improving this by periodically checking how much disk space is available and triggering expiry if it falls below a certain threshold (e.g., 2 GB, could be configurable). I.e., the effective quota will be the minimum of the configured quota and (cache disk usage + available disk space - 2 GB). This is similar to the approach that BuildStream used before switching to casd (except that BuildStream only checked available disk space once at startup).