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.