join query scanning "toField" docValue [LUCENE-6332]

I want to contribute the subj which should do something like ..`WHERE EXISTS (SELECT 1 FROM fromSearcher.search(fromQuery) WHERE fromField=toField)`. It turns out, that it can be returned by the current method `createJoinQuery(...ScoreMode.None)`
- at first, it should run `fromQuery` first, collect `fromField` into `BytesRefHash` by `TermsCollector`, like it's done now
- then it should return query with _TwoPhase_ Scorer
- which obtains `toField` docValue on `matches()` and check term for existence in `BytesRefHash`

Do you think it's ever useful? if you do, I can bake a patch.
Anyway, suggest the better API eg separate method, or enum and actual name!

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Migrated from [LUCENE-6332](https://issues.apache.org/jira/browse/LUCENE-6332) by Mikhail Khludnev (@mkhludnev), updated Mar 12 2015
Attachments: [LUCENE-6332.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6332/LUCENE-6332.patch)
Linked issues:
- [SOLR-6234](https://issues.apache.org/jira/browse/SOLR-6234)

Start at createJoinQuery(..., ScoreMode.None) and trace the existing TermsCollector path that collects fromField into BytesRefHash. Then evaluate the proposed TwoPhase Scorer, including toField doc values and term-existence checks; done means the join query matches documents whose toField exists in the collected fromField terms, with an agreed API.

Written by the indexing model from the issue text.