* Use spandoc correctly
* Refactor consensus test error handling
* Delete a checkpoint test that will soon be obsolete
* Only initialise tracing once for the block tests
* Use tracing in the checkpoint tests
* Move BlockVerifier and tests into block.rs
* Update a BlockVerifier comment
* Tweak some TODO comments
consensus: Add a checkpoint verifier stub
This stub only verifies blocks whose hashes are in the checkpoint
list.
It doesn't have tests, chain child verifies to their ancestors, or
support checkpoint maximum height queries.
Part of #429.