Include estimated active compaction remaining write size when starting a new compaction

There are a few things we can augment in how we handle compactions locally to improve upon the very simple "keep X Mebibytes free on disk" we currently have:

1. Allow specification of free space available in percent rather than raw size
2. Do this on a per-disk basis based on what's going on with compactions
3. Include an estimate of the disk space of active compactions in flight when we do the above calculation check so we don't blow past our limits due to concurrent runs