BP Reordering Codec

[msokolov]

This is a stab at implementing the previously-published BpVectorReorderer as part of the codec. The version we have now can be used as a merge policy to reorder docids by doing BP over a vector field. This version allows for the ordering to be done at the vector ordinal level for each field independently while docids may be soprted by some other IndexSort.

The code is fully working as far as I can tell, and I think the design is reasonable, but some TODOs and nocommits remain:

1. Test results have so far been underwhelming, and I plan to continue to work to see if it is producing results thatare on par with the merge policy version. There might be a bug somewhere, and if anyone is interested in looking, I'd appreciate feedback.
2. There isn't a convenient way to enable/disable reordering in tests. Currently a few tests will fail (the TestBpVectorReorder in particular since it is reordering both at the codec level and separately in the merge policy.