message: Wait to send MTU probes until the handshake is complete
  lsquic failure in interop seems transient, for reference, the error is:
  ```
  22:49:20.238 [ERROR] [QUIC:5233DEFCA107F679] conn: Abort connection: generating ACK frame failed: 22
  ```
diff: @@ -636,10 +636,11 @@ impl<Config: endpoint::Config> connection::Trait for ConnectionImpl<Config> {
  let mut outcome = transmission::Outcome::new(PacketNumber::default());
  let path_id = self.path_manager.active_path_id();
  - // Send an MTU probe if necessary
  + // Send an MTU probe if necessary and the handshake has completed
  // MTU probes are prioritized over other data so they are not blocked by the
  // congestion controller, as they are critical to achieving maximum throughput.
  - if self.path_manager.active_path().can_transmit(timestamp)
  + if self.state == ConnectionState::Active
  + && self.path_manager.active_path().can_transmit(timestamp)
  && self
  .path_manager
  .active_path()
  