feat(iroh,iroh-relay)!: Use stride instead of custom split protocol, send ECN bits (#3389)

## Description

Based on #3331 

- Enable doing active queue management in relays in the future (we now
send ECN bits with messages!)
- Avoid doing our own packet merging & splitting, instead just forward
quinn's `Transmit` format (bytes + optional stride)

## Breaking Changes

The relay wire protocol changed: All relayed messages now contain at
least an additional ECN byte.
They *might* be accidentally compatible when GSO is not enabled, but
they're likely not.
This means this version of iroh can't connect to older relays or older
clients on newer relays.

- `ClientToRelayMsg::SendPacket` was removed in favor of
`ClientToRelayMsg::Datagrams`
- `RelayToClientMsg::ReceivedPacket` was removed in favor of
`RelayToClientMsg::Datagrams`
- `FrameType` has changed variants:
  - `SendPacket` and `RecvPacket` were removed
  - `ClientToRelayDatagram` and `ClientToRelayDatagramBatch` were added
  - `RelayToClientDatagram` and `RelayToClientDatagramBatch` were added

## Change checklist
<!-- Remove any that are not relevant. -->
- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.
- [x] List all breaking changes in the above "Breaking Changes" section.

Author: matheus23

iroh-relay/src/client/conn.rs

@@ -145,10 +145,10 @@ impl Sink<ClientToRelayMsg> for Conn {
     }
 
     fn start_send(mut self: Pin<&mut Self>, frame: ClientToRelayMsg) -> Result<(), Self::Error> {
-        if let ClientToRelayMsg::SendPacket { packet, .. } = &frame {
-            let size = packet.len();
-            snafu::ensure!(size <= MAX_PACKET_SIZE, ExceedsMaxPacketSizeSnafu { size });
-            snafu::ensure!(size != 0, EmptyPacketSnafu);
+        let size = frame.encoded_len();
+        snafu::ensure!(size <= MAX_PACKET_SIZE, ExceedsMaxPacketSizeSnafu { size });
+        if let ClientToRelayMsg::Datagrams { datagrams, .. } = &frame {
+            snafu::ensure!(!datagrams.contents.is_empty(), EmptyPacketSnafu);
         }
 
         Pin::new(&mut self.conn)

iroh-relay/src/protos/common.rs

@@ -23,10 +23,14 @@ pub enum FrameType {
     ServerConfirmsAuth = 2,
     /// The server frame type for authentication denial
     ServerDeniesAuth = 3,
-    /// 32B dest pub key + content
-    SendPacket = 4,
-    /// 32B src pub key + content
-    RecvPacket = 6,
+    /// 32B dest pub key + ECN bytes + one datagram's content
+    ClientToRelayDatagram = 4,
+    /// 32B dest pub key + ECN byte + segment size u16 + datagrams contents
+    ClientToRelayDatagramBatch = 5,
+    /// 32B src pub key + ECN bytes + one datagram's content
+    RelayToClientDatagram = 6,
+    /// 32B src pub key + ECN byte + segment size u16 + datagrams contents
+    RelayToClientDatagramBatch = 7,
     /// Sent from server to client to signal that a previous sender is no longer connected.
     ///
     /// That is, if A sent to B, and then if A disconnects, the server sends `FrameType::PeerGone`

iroh-relay/src/protos/handshake.rs

@@ -418,7 +418,7 @@ pub(crate) async fn serverside(
                 .build()
             })?;
 
-        if let Ok(()) = client_auth.verify(io) {
+        if client_auth.verify(io).is_ok() {
             trace!(?client_auth.public_key, "authentication succeeded via keying material");
             return Ok(SuccessfulAuthentication {
                 client_key: client_auth.public_key,