Let Collector know when all docs have been collected [LUCENE-4370]

Collectors are a good point for extension/customization of Lucene/Solr, however sometimes it's necessary to know when the last document has been collected (for example, for flushing cached data).
It would be nice to have a method that gets called after the last doc has been collected.

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Migrated from [LUCENE-4370](https://issues.apache.org/jira/browse/LUCENE-4370) by Tomas Eduardo Fernandez Lobbe (@tflobbe), 3 votes, updated May 16 2014
Attachments: [LUCENE-4370.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4370/LUCENE-4370.patch) (versions: 2)

Start by reviewing Lucene's collector extension points and the attached LUCENE-4370.patch, then trace how collection completes for a search. Confirm the intended callback timing for the final document and verify that the behavior supports flushing cached data after all documents have been collected.

Written by the indexing model from the issue text.