Upgrade nudge

[brice-stacks]

To maintain decentralization, we don't want some kind of central server that a stacks-node checks for when there are updates, but there is already information available on the decentralized network that can help here. Miners and signers broadcast their version information with every StackerDB message. A node could listen to these and take some action to nudge the node-runner to upgrade. The simplest thing to do would just be to periodically log an error message that might get noticed, but maybe we can come up with other options as well.

[brice-stacks]

From Jude:
how about this?

• we use some of the bytes in the "additional data" field of the p2p preamble to indicate the peer version
• we expand /v2/neighbors to report the remote peer's reported version
• we (optionally) add a query option to /v2/neighbors to get an aggregate view -- e.g. what fraction of peers run which version