Add back snow accepter queue

[aaronbuchwald]

Adds back the async accepter queue into the snow package.

This moves potentially heavy-weight operations during Accept from blocking the consensus thread to a separate async accepter. As this code moves around a few pieces, I also added some improved comments around subscription guarantees (so that we are clear about reliable notifications #1145). This guarantees at least once delivery of block accept notifications.

This adds a new field lastProcessedBlock, which is the last block that the snow package used the chain to mark as accepted and produce an instance of the accepted block type.

A few extra notes on this PR:

• adds SyncAccept to the snow block type, which accepts a block and waits for async work on that block to complete (ensure "effects" of Accept occur before tests check for them)
• There's no clear way for an async thread on an AvalancheGo VM to signal that it a fatal error. To handle fatal errors on the async accepter thread, we use RecoverAndPanic from AvalancheGo's logger and also make sure to call Shutdown on the VM, so we clean up resources before propagating the panic (open to alternative suggestions on the best way to signal fatal here)