Facet benchmarking [LUCENE-3262]

A spin off from #4152. We should define few benchmarks for faceting scenarios, so we can evaluate the new faceting module as well as any improvement we'd like to consider in the future (such as cutting over to docvalues, implement FST-based caches etc.).

Toke attached a preliminary test case to #4152, so I'll attach it here as a starting point.

We've also done some preliminary job for extending Benchmark for faceting, so I'll attach it here as well.

We should perhaps create a Wiki page where we clearly describe the benchmark scenarios, then include results of 'default settings' and 'optimized settings', or something like that.

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Migrated from [LUCENE-3262](https://issues.apache.org/jira/browse/LUCENE-3262) by Shai Erera (@shaie), updated Oct 09 2011
Attachments: [CorpusGenerator.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3262/CorpusGenerator.java), [LUCENE-3262.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3262/LUCENE-3262.patch) (versions: 3), [TestPerformanceHack.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3262/TestPerformanceHack.java)

Start by reviewing the attached CorpusGenerator.java, TestPerformanceHack.java, and LUCENE-3262.patch, then inspect how Lucene's faceting benchmark support is currently structured. Define representative faceting scenarios and document the benchmark cases, including default and optimized settings and their results, as proposed in the issue.

Written by the indexing model from the issue text.