Add aquatic_load_tester: multi-run multi-implementation load tests

- Work in progress
- Only UDP is currently implemented so far
- Also includes some changes to other crates, notably deriving
  serde Serialize for Config structs and making udp_load_test
  a lib and a binary

crates/udp_load_test/src/worker/mod.rs

@@ -41,7 +41,7 @@ pub fn run_worker_thread(
         .register(&mut socket, token, interests)
         .unwrap();
 
-    let mut events = Events::with_capacity(config.network.poll_event_capacity);
+    let mut events = Events::with_capacity(1);
 
     let mut statistics = SocketWorkerLocalStatistics::default();