Parallel incremental indexing [LUCENE-1879]

A new feature that allows building parallel indexes and keeping them in sync on a docID level, independent of the choice of the MergePolicy/MergeScheduler.

Find details on the wiki page for this feature:

http://wiki.apache.org/lucene-java/ParallelIncrementalIndexing

Discussion on java-dev:

http://markmail.org/thread/ql3oxzkob7aqf3jd

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Migrated from [LUCENE-1879](https://issues.apache.org/jira/browse/LUCENE-1879) by Michael Busch, 5 votes, updated May 09 2016
Attachments: [parallel_incremental_indexing.tar](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-1879/parallel_incremental_indexing.tar)
Linked issues:
- #3100
- #3101
- [SOLR-139](https://issues.apache.org/jira/browse/SOLR-139)

Start with the ParallelIncrementalIndexing wiki page, the java-dev discussion, and the attached parallel_incremental_indexing.tar. Review linked issues #3100, #3101, and SOLR-139 to understand the intended scope and integration points. Done means implementing and validating parallel indexes that stay synchronized at the docID level regardless of MergePolicy or MergeScheduler.

Written by the indexing model from the issue text.