PreLaunch Testnet - [Release process] Store mock-mined blocks and replay them

[jcnelson]

The bug fixed by #4361 could have been caught if, as part of our release process, we (1) ran the mock-miner for a long-ish duration (e.g. 1 week), (2) stored all the blocks it produced, and (3) replayed them against the current release to ensure that they are all valid. To address this, we'd need to:

• Update the mock-miner to store its blocks (preferably in a separate DB or directory)
• Merge the branch test/replay-block and ilk to master
• Extend test/replay-block so that it can take a block that does not yet exist in the chainstate (e.g. from the mock miner)