After network upgrade activation, close existing connections to outdated peers

[teor2345]

Motivation

zcashd automatically disconnects from outdated peers after a network upgrade activates.

Specifications

Original specification: ZIP 201: Network Peer Management for Overwinter

Since this is a network protocol specification, Zebra is free to adopt any compatible design.

Draft Zebra behaviour: ZIP 252: Deployment of the NU5 Network Upgrade - Support in Zebra

Solution

Peer Set:

• Add a ReadyService type containing a Discover::Service peer service
• Track the service connection's network protocol version in the ReadyService and UnreadyService
• Close any open connections from outdated peers when the min_peer_version increases
  • security: rate-limit these connection closes to 1 per second, to avoid connection floods and temporary starvation
    • this rate-limit can be implemented in PeerSet::poll_ready

Documentation:

• Update ZIP 252: Deployment of the NU5 Network Upgrade - Support in Zebra

Testing:

• Sync past the testnet activation height, and check Zebra's behaviour
• Run and monitor Zebra nodes on mainnet during activation
• TODO: what other tests can we do here, without too much effort?

Alternatives

We could rely on natural connection churn to get rid of these peers. But it doesn't provide any guarantees.

We could do #2813 after NU5 mainnet activation, but it would only apply to updated Zebra instances.

Related Work

Depends on #1334: Automatically reject new connections from outdated peers after network upgrade activation

[teor2345]

The AddressBook-based design was wrong, because the address book only tracks outbound peer connections.

I replaced it with a peer-set based design.