Introducing AppProtocolVersion.NetworkId

[dahlia]

In order to prevent two different networks (e.g., mainnet and testnet) from being mixed up, we need to add a separate network-level (or app-level?) field to indicate the network identifier to connect to. However, as it would be quite difficult to add a new property/parameter to existing data types and functions taking APV, it might be easier to add the new component to AppProtocolVersion.

This new component should be prioritized over the version number when two APVs are compared, so that a greater version number with a different network ID does not trigger the update event.

[dahlia]

Unlike other components of AppProtocolVersion, maybe NetworkId shouldn't be signed so that it's easily switched from other network without building distinct binaries for each network or allowing the entire APV to be configured:

AppProtocolVersion apv = new AppProtocolVersion("it should be hard-coded in the app code");
if (config.Testnet)
{
    AppProtocolVersion apv = apv.WithNetworkId("testnet");
}