Remove dependence on rsync

While rsync was a quick way to initially implement Dave, it requires `rsync` to be installed on the machine alongside Dave. In a container, this requires us to include an `rsync` binary.

If we could remove the dependence on Rsync, it would make dave a lot less error prone and far more portable.

The issue names no files, tests, or entry points. Start by locating where Dave invokes or depends on rsync, then trace the snapshotting path; done means Dave no longer requires an rsync binary while retaining its data snapshotting behavior and improving portability in containers.

Written by the indexing model from the issue text.