Node: Rely on canonical affirmation map to know when to wait for anchor blocks

[jcnelson]

This supersedes https://github.com/blockstack/stacks-blockchain/issues/2095.

• We should update the Burnchain::sync() method to first compute the canonical affirmation map. This would mean downloading all burnchain block ops before downloading Stacks blocks, or possibly receiving it on the command-line as a hint.

• We should remove the PoxSyncWatchdog, and have the main run-loop rely solely on the chains coordinator to block chain synchronization if we're waiting for a known-extant anchor block.

• We may need to tweak the downloader so the run-loop can hint to the downloader that an absent but known-extant anchor block is blocking chain synchronization.

[jcnelson]

Removing from the Stacks 2.1 project for now, since it can be implemented afterwards.