SoV networking support

[StephenButtolph]

Why this should be merged

Currently, the Avalanche network topology is a combination of strongly connected and a hub-spoke networks.

The Avalanche Primary Network validators form a strongly connected network. This enables Snow* consensus to rapidly confirm blocks.

Non-validators also connect to all Primary Network validators (but not other non-valdiators). This ends up looking like the Primary Network acting like the hub and all non-validators acting as spokes.

In order to connect to a Subnet, no additional connectivity logic is required because Subnet validators are required to also be Primary Network validators. This means that by just re-using the Primary Network's peer discovery, the Subnet will also have its own strongly connected validator hub and non-validator spokes.

However, ACP 13 and later ACP 77 suggest to remove the requirement that Subnet validators must also be Primary Network validators. In order to remove the restriction we must extend the peer discovery mechanism to understand Subnets.

How this works

The PR extends the IPs that Primary Network validators are tracking to include all Subnets. Subnets will utilize the Primary Network to bootstrap their connections.

When peers connect they already communicate the Subnets they are tracking. When a Primary Network validator is sent a GetPeerList message, they will respond to the request for IPs that are validators of Subnets that are tracked by the requester.

How this was tested

• Unit Tests
• CI
• Modifying the node to support SoV and verifying that a subnet without primary network validators can be efficiently connected to and synced.

[marun]

The code changes LGTM. The test changes are so big though.