Add option to remove (oldest) snapshot when space is low

Unless I'm missing something, btrbk has no (configurable) check as to the free space on the host nor the target. An option to remove the oldest snapshot if the free-space is lower than say 2% would be a nice addition.

The issue names no files, tests, or entry points. First identify how free space is measured on the host and target and where the oldest snapshot is selected for deletion. Done means a configurable low-space threshold causes the intended oldest snapshot to be removed safely, with tests covering the behavior.

Written by the indexing model from the issue text.