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Add updates for merge changes in Accumulo 4.0 #452

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Adds some initial information for users for the updated merge changes in Accumulo 4.0. I didn't want to go to detailed into the internals but the changes describe how the new merge is different (faster and fences files, etc compared to chop compactions) and also goes into auto merge and how to configure tablets using the API. I'm marking it as a draft as I'm sure some changes are needed and polishing but want to get some feedback on how things look and if we need more detail or not. I also assume there will be updates in the future before release as things are tweaked as 4.0 still is not close to release.


* `manager.tablet.mergeability.interval` -Time to wait between scanning tables to identify ranges of tablets that can be auto-merged (default is `24h`)
* `table.mergeability.threshold` - A range of tablets are eligible for automatic merging until the combined size of RFiles reaches this percentage of the split threshold. (default is `.25`)
* `table.merge.file.max` - The maximum number of files that a merge operation will process (default is `10000`)
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This property applies to both api initiated merges and automatically initiated merges, the other two props only apply to automatic merges. Not sure if this is worth working in somehow, it was just something I noticed.

* `table.mergeability.threshold` - A range of tablets are eligible for automatic merging until the combined size of RFiles reaches this percentage of the split threshold. (default is `.25`)
* `table.merge.file.max` - The maximum number of files that a merge operation will process (default is `10000`)

## Tablet Mergeability
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May want to work in to this section what will happen on upgrade for existing tablets.

@cshannon cshannon marked this pull request as ready for review March 27, 2025 19:32
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