Trigger compaction based on store size

[vgkholla]

Explore ways to trigger compaction based on the how "full" the store is. There should be a mechanism to request compaction (akin to how admin requests to trigger compaction are made) when the store discovers that it is past a certain fill size.

One of the things to keep in mind is that compaction should not get triggered often, a store must be able to determine either if a compaction request will be useful (might be undesirable since it might redo work that compaction will do) or that it is not repeating a request that yielded no results in a previous call.

This also works well in conjunction with #737