Statistics from ValueSourceQuery [LUCENE-989]

Patch forthcoming that adds a DocValuesStats object that is optionally computer for a ValueSourceQuery. This \~replaces the getMin/Max/Avg from the DocValues which were previously unaccessible without reasonably heavy subclassing. in addition it add a few more stats and provides a single object to encapsulate all statistics going forward. The stats object is tied to the ValueSourceQuery so that the values can be cached without having to maintain the full set of DocValues. Test and javadocs included.

- will

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Migrated from [LUCENE-989](https://issues.apache.org/jira/browse/LUCENE-989) by Will Johnson, updated May 09 2016
Attachments: [functionStats.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-989/functionStats.patch)
Linked issues:
- #2162

Start by reviewing the attached functionStats.patch and the existing ValueSourceQuery and DocValues APIs. Check the tests and Javadocs included with the patch to understand the intended statistics and caching behavior. Done means the proposed DocValuesStats functionality is covered by tests and documented.

Written by the indexing model from the issue text.