Triggered one bug in ΔFtail.SliceByFileRev which was not checking
file's root to be VDEL.

Knowing which key coverage is being tracked will be needed to implement
ΔBtail concurrency.

This will be needed to organize concurrent rebuild/queries, so that
several rebuild jobs could be run in parallel, and only when they are
done, one particular place of code locks ΔBtail/ΔTtail and merges
rebuild results into vδT & friends.

neo@20e8f37b