Make the root to sync more flexible

[gideonthomas]

Currently the root to sync is a little unstable as we allow anything on it. For e.g. we allow deleting the sync root (/dir) on one client and this does not change its sync root. This causes problems where this root might be a part of another client (whose root is /dir/myroot) and is trying to downstream changes.

We need a solution to handle deletion of the sync root so that we can regress to the parent without disrupting anything.