Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/client/network/src/protocol_controller.rs

@@ -493,8 +493,8 @@ impl ProtocolController {
 		}
 	}
 
-	/// Remove the peer from the set of reserved peers. The peer is moved to the set of regular
-	/// nodes.
+	/// Remove the peer from the set of reserved peers. The peer is either moved to the set of
+	/// regular nodes or disconnected.
 	fn on_remove_reserved_peer(&mut self, peer_id: PeerId) {
 		let state = match self.reserved_nodes.remove(&peer_id) {
 			Some(state) => state,
@@ -508,7 +508,14 @@ impl ProtocolController {
 		};
 
 		if let PeerState::Connected(direction) = state {
-			if self.reserved_only {
+			// Disconnect if we're at (or over) the regular node limit
+			let disconnect = self.reserved_only ||
+				match direction {
+					Direction::Inbound => self.num_in >= self.max_in,
+					Direction::Outbound => self.num_out >= self.max_out,
+				};
+
+			if disconnect {
 				// Disconnect the node.
 				trace!(
 					target: LOG_TARGET,