Milestone 1 for Nakamoto workstream: #3855

Goal: bring the Nakamoto node online.

This milestone would see the following functionality implemented:

• There is a single miner, and no stacker. It produces one block about once every five seconds.
• It runs against a Bitcoin regtest node, which produces Bitcoin blocks once every 60 seconds.
• The miner sends the new style of block-commits. The block header hash field contains the StacksBlockId of the last tenure’s first block.
• The TenureChange transaction is synthesized by the miner, but is present in the block stream.