Divide peers into three categories and apply proper limitation to them

[Geod24]

Currently, our only check for our peers is as follow:

agora/source/agora/network/Manager.d

Lines 84 to 88 in 14459dd

	///
	public bool isValidator () const scope @safe pure nothrow @nogc
	{
	return this.key != PublicKey.init;
	}

However, this check is incorrect: First, any node configured as a validator will have a public key, regardless of its enrollment status:

agora/source/agora/node/Validator.d

Lines 337 to 341 in 14459dd

	auto enroll_key = this.enroll_man.getEnrollmentKey();
	Identity id = Identity(this.config.validator.key_pair.address,
	enroll_key == Hash.init ? this.getFrozenUTXO() : enroll_key);
	if (key == PublicKey.init)
	return id;

What we should do is check for the utxo not being Hash.init, and compare this against the Ledger.
However, that would only leave us with two kinds of peers: full nodes and validators. The list of validators is not very stable by definition, and we've been moving towards using the presence of a stake as a criteria, rather than the enrollment itself, e.g. #2859

The same should probably be done for the network manager, so that it can differentiate nodes with identities and nodes without.