Add query time parent child search [LUCENE-3938]

At the moment there is support for index time parent child search with two queries implementations and a collector. The index time parent child search requires that documents are indexed in a block, this isn't ideal for updatability. For example in the case of tv content and subtitles (both being separate documents). Updating already indexed tv content with subtitles would then require to also re-index the subtitles.

This issue focuses on the collector part for query time parent child search. I started a while back with implementing this. Basically a two pass search performs a parent child search. In the first pass the top N parent child documents are resolved. In the second pass the parent or top N children are resolved (depending if the hit is a parent or child) and are associated with the top N parent child relation documents. Patch will follow soon.

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Migrated from [LUCENE-3938](https://issues.apache.org/jira/browse/LUCENE-3938) by Martijn van Groningen (@martijnvg), updated May 07 2012
Attachments: [LUCENE-3938.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3938/LUCENE-3938.patch) (versions: 2)

Start by reading the issue description and the attached LUCENE-3938.patch, focusing on the proposed collector and its two-pass query-time parent-child search. Done means the collector resolves the top N parent-child documents in the first pass and associates the selected parent or child documents in the second pass.

Written by the indexing model from the issue text.