- Reimplement ΔFtail queries via gluing ΔBtail and ΔZtail data on the fly.
  This helps to avoid implementing complex rebuild logic in ΔFtail.
  The only place that needs to have that complexity is now ΔBtail, and there it
  already works draftly.

- Add ΔFtail tests.

- Add notion of epochs to ΔFtail. Epochs correspond to ZBigFile objects changes
  (creation and deletion). Unfortunately handling ZBigFile object changes
  turned out to be necessary to keep wcfs tests in passing state.

- Move common testing infrastructure - that is used by both ΔBtail and ΔFtail - to xbtreetest package.

- Add tests for ΔBtail.SliceByRootRev aliasing

- Lazy rebuild is now on

- ΔBtail.GetAt reworked

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* t2: (112 commits)
  X wcfs: v↑ NEO/go (checkpoint)
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  X ΔFtail: Rebuild vδE after first track
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To pick up neo@bc3b5ec3

This is necessarry because previously both ΔFtail.BlkRevAt and
ΔBtail.GetAt required zfile/root checkouts from @head. However during
update process in wcfs it is ΔFtail that is updated first, and only
after some invalidations zconn is resynced to new @head. And if we
move zconn.resync to happen before ΔFtail.Update, that would break
the invariant that data in OS cache correspond to zconn.At.

The rework triggered and clarified logic of what ΔBtail.GetAt
responsibility and functionality should be. Now it computes query result
only based on ΔBtail data, and returns valueExact=false or
revExact=false, if those data is insufficient. Previously it was further
trying to load data from the database which is conflicting with

	// ΔBtail semantically consists of
	//
	//	[]δB			; rev ∈ (tail, head]

Now that database access moved to GetAt user - to ΔFtail.BlkRevAt.

WCFS tests - that were broken after ΔFtail rework - now finally pass again.

* t2+qoldhead:
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Unfortunately I could not avoid computing diff for ZBigFile objects
themselves, because after recent ΔFtail and ΔZtail fixes, wcfs tests
started to fail becase in the following scenario

    δFtail is created                        @at0
    file is created                          @at1
    file starts to be tracked when δFtail is @at2
    δFtail query is made about file block -> boom reports @at0, because the file does not exist there at all

To fix this ΔFtail now detects when ZBigFile objects are changed
themselves and indicate such a change with specifal δf with
δf.Epoch=true .

* t2+δfepoch: (38 commits)
  X ΔFtail: Rebuild vδE after first track
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This is needed to detect where the file was created and indicate with
δF.Epoch=true correspondingly.