dataplane, transport: node keys -> peer ids

[npry]

Begin using ts_transport::PeerId (a locally-assigned peer index allocated by ts_runtime::PeerTracker) to uniquely identify peers. This lets us stop referring to NodePublicKey for peer identification from everywhere except ts_tunnel and derp (which need it to function). ts_tunnel now is no longer responsible for allocating peer ids, and instead is informed what peer id to use for what node key. (That happens in DataplaneActor as part of the peer update received from PeerTracker.)

This change revolves around PeerDb, which is HashMap<PeerId, Node> augmented by a number of secondary indices on nodekey, name, stable id, id, tailnet ips, accepted routes, and disco key (if known).

Implements an in-place update of the node key in wireguard for a given PeerId if the nodekey changes.

Closes #167 -- the PeerDb's name, ip, and route indexes address this

underlay transport

The last two commits refactor ts_transport::UnderlayTransport and derp's implementation of it. First, I wrote a helper trait that captures the packet batch iterator idea (this is basically what we had discussed @dylan-tailscale, I found a way to represent it that I think is actually easier to read, and also we need it on the recv side now too, so it got repetitive).

But the bigger change is that UnderlayTransport now takes a PeerKey associated type, which is PeerId inside ts_runtime, but NodePublicKey inside derp. I also provide a with_lookup combinator that allows mapping the key type for a transport using a PeerLookup trait, which the runtime implements by referencing the PeerDb. This lets derp and any other underlay transport not care about the peer id mapping, since the runtime handles the translation to/from the transport-specific peer identifier.

This is what that looks like in ts_runtime::Multiderp:

tailscale-rs/ts_runtime/src/multiderp.rs

Lines 131 to 152 in e7dc788

	struct PeerDbLookup<'a>(&'a RwLock<Option<Arc<PeerDb>>>);
	impl ts_transport::PeerLookup<PeerId, NodePublicKey> for PeerDbLookup<'_> {
	fn lookup_key(&self, id: PeerId) -> Option<NodePublicKey> {
	let db = self.0.read().unwrap();
	let db = db.as_ref()?;
	let (_, node) = db.get(&id)?;
	Some(node.node_key)
	}
	}
	impl ts_transport::PeerLookup<NodePublicKey, PeerId> for PeerDbLookup<'_> {
	fn lookup_key(&self, key: NodePublicKey) -> Option<PeerId> {
	let db = self.0.read().unwrap();
	let db = db.as_ref()?;
	let (id, _) = db.get(&key)?;
	Some(id)
	}
	}

tailscale-rs/ts_runtime/src/multiderp.rs

Lines 187 to 188 in e7dc788

	let client = ts_derp::DefaultClient::connect(&region.servers, &keys).await?;
	let transport = client.with_key_lookup(PeerDbLookup(peer_db));

client above implements UnderlayTransport<Key = NodePublicKey>, and transport wraps it using the PeerDbLookup to provide UnderlayTransport<Key = PeerId>

review notes

Review commits one at a time — the second commit is a pure rename