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

This data structure will be used to implement concurrency in ΔBtail.

* t2+RangedMap: (45 commits)
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  X fixes for empty set/range
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  X Tests for Get
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  X SetRange draftly works
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  X need to keep Value first so that sizeof(set-entry) = sizeof(KeyRange)
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  ...

see github.com/golang/go/issues/48651