Use the CandidateSet and AddressBook to connect to initial seed peers

[teor2345]

Motivation

Zebra has custom code that connects to all its initial seed peers on startup.

But we could use the AddressBook and CandidateSet to connect to these peers.

This would make the code simpler and more reliable. But we would need to maintain some existing security and reliability features of Zebra.

Specifications

When started for the first time, programs don’t know the IP addresses of any active full nodes. In order to discover some IP addresses, they query one or more DNS names (called DNS seeds) hardcoded into Bitcoin

https://developer.bitcoin.org/devguide/p2p_network.html#peer-discovery

Solution

Type Refactor

• Add MetaAddrChange::NewSeed and PeerAddrState::NeverAttemptedSeed variants
  • these variants only have a SocketAddr
  • security: these variants must not be assigned the current time - having a None time makes sure Zebra checks their addresses before gossiping them (existing behaviour)

Function Refactor

• In add_initial_peers, send the peers to the AddressBook
  • security: the CandidateSet will make sure that initial peer connections are rate-limited (originally fixed by Security: Rate-limit initial seed peer connections #2326)
• Make sure that the first connected peer gets an immediate Addrs request
  • reliability: on small networks and with small seed sets, Zebra must get more addresses from this peer to reliably connect to other nodes (existing behaviour)

Testing

• Test Zebra with a single seed peer
  • add an acceptance test that launches a Zebra node with the default initial peers config
  • also launch a Zebra node with only that peer's inbound listener in its initial peers config
  • check they both sync genesis and connect to multiple other peers
• Test Zebra launch reliability using the existing acceptance tests

Alternatives

We could implement the initial peers rate-limit by waiting for a short time between each initial peer connection, see #2326.

But the rest of this change is a refactor and cleanup.

[teor2345]

This would be a useful simplification, but it's not required.