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.

*ZBigFile is live object associated with particular ZODB Connection.

- gen-δtail is not used - we are building ΔFtail from scratch.
- gogcccrash did its work and the problem is now understood.

* t:

Now both normal and random tests for all Update and rebuild could be
exercised as part of regular test runs.

* t2:
  X rebuild: tests: Random testing
  X rebuild: tests: Don't exercise keys from keys2 that already became tracked after Track(keys1) + Update
  X rebuild: tests: Inline _assertTrack
  X rebuild: tests: Don't compute keyCover in trackSet
  X rebuild: tests: Don't recompute trackSet(keys1R2) several times
  X rebuild: tests: Move ΔBtail.Clone test out of hot inner loop into separate test
  X tests: Factor-out tree-test-env into tTreeEnv
  X xbtree: Less copy/garbage in RangedKeySet ops
  X rebuild: tests: Precompute kadj10·kadj21
  X rebuild: tests: Don't access ZODB in xtrackKeys
  X rebuild: tests: Don't access ZODB in XGetδKV
  X rebuild: tests: Don't reflect.DeepEqual in inner loop
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Rework py allstructs to emit just set of random trees for one kv.
Rework TestΔBTailAllStructs to iterate through such obtained sets and
generate all pairs and triplets. Previously it was only pairs which is
not enough to verify rebuild properly.

X rebuild: tests: Don't exercise keys from keys2 that already became tracked after Track(keys1) + Update

58s -> 14s  (in -short mode)

t.Helper() is relatively expensive

61s -> 58s

63s -> 61s

68s -> 63s