B+tree discussions

[Robert Collins]

At this point, I think we have a 'good enough to be an improvement'
candidate, once some remote testing is done. (And I'm on that now).

B+Tree, no bloom, with 1000 page cache outperforms or is equal with
GraphIndex on every benchmark I've come up with. I think in real world
larger-still indices it should be tolerable if not better due to lower
memory consumption, and it should be tunable (simply by upping the cache
size) to deal with such cache thrashing workloads.

I think bloom use is a compelling future feature, but its not a clear
enough win /today/ and we can take the time to make it better later.

If everyone agrees with this concept, I'll make the stackable formats
about to be merged, also include this index layer.

-Rob
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