This way ΔFtail tests will be able to reuse it.

* t3:
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  Revert "."
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This reverts commit a5027dc7.

* t2:
  X Unexport SetXXX from packages API

* t2:
  X No longer rely on ZODB cache invariant for invalidations
  X ΔFtail: Keep reference to ZBigFile via Oid, not via *ZBigFile

Move all aspects of what is tracked into ΔFtail.
It is more robust if ΔFtail does not rely on properties of outside
zconn.Cache in order to work correctly.
It also potentially allows to move ΔFtail out of zdata package.

And it is even more memeory efficient, as before it was

	Connection.cache[zoid] -> ZBlk(ghost)->inΔFtail

and now it is

	ΔFtail.trackSetZBlk[zoid] -> zblkTrack

becuase Persistent embedded in ZBlk even in ghost state occupies alone
occupies ~ 16 words.

Now we keep only what is needed to be kept for tracking.