Reduce the amount of time that the block verifier waits for the previous
block, before polling again.
(Waiting for 2 seconds resulted in some apparent block verifier hangs.)
This is a temporary fix, until the state layer handles context checks.
* Flatten consensus::verify::* to consensus::*
* Move consensus::*::tests into their own files
* Move CheckpointList into its own file
* Move Progress and Target into a types module
QueuedBlock and QueuedBlockList can stay in checkpoint.rs, because
they are tightly coupled to CheckpointVerifier.