Add FacetRequest.ResultMode.AUTO_EXPAND [LUCENE-4767]

A ResultMode.AUTO_EXPAND will auto-expand a node in tree if only one result came up. For example, if you have a Date facet, and you ask to count the top-K of Date/, then if your documents were all created in say 2010, you'll get a redundant facet back: Date/2010 (no other children of Date/).

Rather, since we already have all the categories counted at hand, we can "auto-expand" Date/2010 recursively, until we get to a level in the tree which has more than one children. And of course, if all the results belong to a single leaf node, then we should return it ...

This can be implemented by adding a new ResultMode and a special FacetResultsHandler, which performs the auto-expansion. I think it should be limited to depth=1 only at the moment ...

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Migrated from [LUCENE-4767](https://issues.apache.org/jira/browse/LUCENE-4767) by Shai Erera (@shaie)

Start by locating FacetRequest.ResultMode and the FacetResultsHandler implementation. Trace how top-K facet results are produced, then determine how a depth=1 AUTO_EXPAND mode would recursively follow a sole child until a level has multiple children or reaches a leaf. Add focused coverage for the Date/2010-style single-child and multi-child cases, with completion shown by the expected expanded results.

Written by the indexing model from the issue text.