[Network] assist Bitcoin block relay

[jcnelson]

We've noticed that when running a collection of Bitcoin regtest nodes, it can take on the order of minutes for all of the nodes to sync their chains (despite the blocks being almost empty, and despite all peers knowing about each other).

The Stacks p2p network could remedy this by optionally relaying Bitcoin nodes to peers that report an older burnchain view. Then, their Bitcoin nodes could be kept in sync.