Automatic merging strategies

When significant data is deleted from consecutive tablets, they could be merged and still fall under the configured split threshold. Is systems where data is date oriented and is aged off, a significant number of tablets that are empty or near-empty can be left around over time resulting in a bloat of tablets. We need to allow the system to automatically merge tablets. A configurable merging strategy interface should be used to allow systems to configure how they want this to happen over time.

The issue names no files, tests, or entry points. Begin by locating the tablet split-threshold and tablet-management implementation, then determine how a configurable merge strategy would fit; done means the system can automatically merge eligible consecutive tablets according to a configurable strategy.

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