No hook provided in IndexWriter.java before merge after setup, for auxiliary indexes [LUCENE-3704]

When trying to manage an auxiliary index to lucene, IndexWriter.java 3.5.0 does not provide a proper hook. Such a hook (see attached proposed patch) permits us to manage an outside index where we get 1 of our segments in parallel to the lucene segments, and we can keep the same docid offsets within the segment.

We are attaching a suggested patch that provides an override-able method. We would like this accepted for 3.5+ if possible. Thanks.

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Migrated from [LUCENE-3704](https://issues.apache.org/jira/browse/LUCENE-3704) by Ken McCracken, 3 votes
Attachments: [bobo-contrib-geosearch.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3704/bobo-contrib-geosearch.patch)

Start with IndexWriter.java and the attached bobo-contrib-geosearch.patch to understand the requested hook after setup and before merge. Done means an override-able hook exists at the required point so an auxiliary index can preserve matching segment document-ID offsets.

Written by the indexing model from the issue text.