Update Luwak for 6.6.1 and/or 7.x

[dcollins53]

Any objections to moving Luwak to use a later Lucene 7.x release?

Tried 6.6.1 which Luwak seems to work fine with out of the box (not sure a release with just that is worth it?)

Also tried 7.0 which I broke down into a number of issues:

• Legacy Numerics (RanqeQuery, TokenStream, etc)
• TermsQuery has been replaced by TermInSetQuery
• changes to createWeight API (addition of boost), no RandomAccessWeight, other Weight API changes
• changes to LeafReader API (no fields)
• changes to DocValue API (no direct getter)

Any thoughts on trying to work on those? I can have a stab where possible...

[romseygeek]

I'm not going to have time to do this immediately, but it's definitely on my list.

There will be a number of new queries to deal with too, and I think we can look at using some of the point range queries to allow the Presearcher to filter out point queries.

[dcollins53]

#166, #167, #168, #169 are some PRs to remove use of classes which are not in Lucene 7, these are intended to be patched onto master (to prove all the functionality still works against Lucene 6) before the actual port to Lucene 7 (which should then just be API tweaks like LeafReader, DocValue, createWeight tweaks)...