MergePolicy simulator utility [LUCENE-8331]

This issue introduces a MergePolicy simulator utility to help evaluate the effectiveness of a MergePolicy.  The simulator does not result in the actual indexing and merging of segments; instead it provides some dummy constructs to MergePolicy to evaluate its decisions. Therefore you can do simulation runs in little time.

I'm not sure where it would live. Perhaps dev-tools, or in tests, or in benchmark?

I mentioned this recently here:
https://issues.apache.org/jira/browse/LUCENE-7976?focusedCommentId=16446985&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16446985

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Migrated from [LUCENE-8331](https://issues.apache.org/jira/browse/LUCENE-8331) by David Smiley (@dsmiley), updated Jan 29 2020
Attachments: [LUCENE-8331.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8331/LUCENE-8331.patch)
Linked issues:
- #10005