aquatic_http: add keep_alive setting (default true)

This might be good for performance in the real world. It also
enables fairer load test comparisons with opentracker, which
in my understanding drops connections after sending responses.

aquatic_http/src/lib/config.rs

@@ -55,6 +55,7 @@ pub struct NetworkConfig {
     pub ipv6_only: bool,
     #[serde(flatten)]
     pub tls: TlsConfig,
+    pub keep_alive: bool,
     pub poll_event_capacity: usize,
     pub poll_timeout_microseconds: u64,
 }
@@ -128,6 +129,7 @@ impl Default for NetworkConfig {
             address: SocketAddr::from(([0, 0, 0, 0], 3000)),
             ipv6_only: false,
             tls: TlsConfig::default(),
+            keep_alive: true,
             poll_event_capacity: 4096,
             poll_timeout_microseconds: 10_000,
         }