Faceting for DateRangePrefixTree [LUCENE-5735]

The newly added DateRangePrefixTree (DRPT) encodes terms in a fashion amenable to faceting by meaningful time buckets. The motivation for this feature is to efficiently populate a calendar bar chart or [heat-map](http://bl.ocks.org/mbostock/4063318). It's not hard if you have date instances like many do but it's challenging for date ranges.

Internally this is going to iterate over the terms using seek/next with TermsEnum as appropriate. It should be quite efficient; it won't need any special caches. I should be able to re-use SPT traversal code in AbstractVisitingPrefixTreeFilter. If this goes especially well; the underlying implementation will be re-usable for geospatial heat-map faceting.

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Migrated from [LUCENE-5735](https://issues.apache.org/jira/browse/LUCENE-5735) by David Smiley (@dsmiley), updated Aug 09 2018
Attachments: [LUCENE-5735__PrefixTreeFacetCounter.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5735/LUCENE-5735__PrefixTreeFacetCounter.patch), [LUCENE-5735.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5735/LUCENE-5735.patch) (versions: 2)
Linked issues:
- #6710
- #7253