CheckIndex: pre-exorcise document id salvage [LUCENE-8961]

The [CheckIndex](https://github.com/apache/lucene-solr/blob/releases/lucene-solr/8.2.0/lucene/core/src/java/org/apache/lucene/index/CheckIndex.java) tool supports the exorcising of corrupt segments from an index.

This ticket proposes to add an extra option which could first be used to potentially salvage the document ids of the segment(s) about to be exorcised. Re-ingestion for those documents could then be arranged so as to repair the data damage caused by the exorcising.

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Migrated from [LUCENE-8961](https://issues.apache.org/jira/browse/LUCENE-8961) by Christine Poerschke (@cpoerschke), updated Oct 01 2019
Attachments: [LUCENE-8961.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8961/LUCENE-8961.patch) (versions: 2)

Start with CheckIndex.java and the attached LUCENE-8961.patch to understand the existing exorcise flow and proposed option. Trace how the CheckIndex tool identifies segments for removal, then define how document IDs would be salvaged before exorcising; done means the option works without changing the existing exorcise behavior.

Written by the indexing model from the issue text.