Refactor CommandsForKey to simplify and reduce size

[belliottsmith]

This patch refactors CommandsForKey to a single efficient special-purpose collection, that can answer all queries without occupying too much memory. There remain further efficiencies to be had.

In addition, this patch introduces the MaxConflicts collection, and moves CommandsForKey to be updated synchronously rather than via Listener updates to support deterministic execution. This will later be moved to a permitted-asynchronous update.