* Make sure the Canopy activation block is a finalized checkpoint block
This enables ZIP-221 chain history from Canopy activation onwards.
* Clarify that the mandatory checkpoint test includes Canopy activation
The test was correct, but the docs and assertion message did not include activation.
* Document that the mandatory checkpoint includes Canopy activation
Co-authored-by: teor <teor@riseup.net>
This reduces the API surface to the minimum required for functionality,
and cleans up module documentation. The stub mempool module is deleted
entirely, since it will need to be redone later anyways.