ValueSourceScorer (used in VS range queries) ought to honor FV.exists [LUCENE-6727]

(discussion split off from LUCENE-6720)
ValueSourceScorer is an abstract scorer based on a ValueSource; notably it's returned by FunctionValues.getRangeScorer. None of the implementations consider FunctionValues.exist(doc). It really ought to. DocValuesRangeQuery does.

Perhaps this should be based on the ValueFillter/MutableValue API. It optimizes existence & value fetching in one fetch when possible. Furthermore, it might be possible to have a common range query check without doing it per-type?

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Migrated from [LUCENE-6727](https://issues.apache.org/jira/browse/LUCENE-6727) by David Smiley (@dsmiley)

Start by tracing ValueSourceScorer and its FunctionValues.getRangeScorer implementations, then compare their existence handling with DocValuesRangeQuery. Review FunctionValues.exists(doc), ValueFilter, and MutableValue to determine how range scoring should exclude non-existent values; done means all relevant range scorers honor existence consistently.

Written by the indexing model from the issue text.