arch-210-silo-write-combiner

[jimmyaxod]

This introduces a component we can use to combine writes from different sources, with different priorities.

Consider a migration. On the destination, if we are using S3 assisted, we essentially end up with 3 sources for writes.

1. Local writes (Should have highest priority)
2. Dirty blocks coming from source p2p
3. Blocks we are grabbing from S3

This component will allow us to reconcile these 3 sources of writes.

This is a more general form of waitingCache, and there is crossover, but that will be addressed in a later PR.