Estimate compaction based on diff to previous compacted head state

Food for thought: try to base the compaction estimation on a diff between the latest compacted state and the current state.

Pros

• estimation duration would be proportional to number of changes on the current head state
• using the size on disk as a reference, we could actually stop the estimation early when we go over the gc threshold.
• data collected during this diff could in theory be passed as input to the compactor so it could focus on compacting a specific subtree

Cons

• need to keep a reference to a previous compacted state. post-startup and pre-compaction this might prove difficult (except maybe if we only persist the revision similar to what the async indexer is doing currently)
• coming up with a threshold for running compaction might prove difficult
• diff might be costly, but still cheaper than the current full diff