add continuous mode

[sbrudenell]

the dream is to have btrfs2s3 operate in "continuous mode".

my thought is to query a source very frequently, possibly every 1-5 seconds, and create snapshots and backups as necessary. this allows us to back up data even more frequently than crontab would allow.

we can even save on ListObjectsV2 calls in this mode, as we can cache the state and only periodically update it from the server.

[sbrudenell]

in particular: btrfs2s3 update is only occasionally long-running, when uploading new full backups. this makes it awkward to run in cron.

we'd probably need lockfiles and timeouts to make btrfs2s3 update more usable, but it would be nice to have btrfs2s3 update be tailored for one-shot interactive runs and continuous mode be non-interactive, rather than add features to btrfs2s3 update so it can work either way.