refactor!: disable keep-alives by default

This changes the `keep_alive_interval` set for `Config::default()` to
`None`, disabling keep-alives. This is thought to be a more sane default
for a library, since keep-alives may otherwise cause connections to stay
open forever, leaking resources. It remains easy to opt-in to
keep-alives by simply setting `Some(duration)`.

BREAKING CHANGE: The `qp2p::config::DEFAULT_KEEP_ALIVE_INTERVAL`
constant has been removed, and the default value for
`keep_alive_interval` set for `Config::default()` is now `None`, meaning
keep-alives are disabled by default.

src/config.rs

@@ -18,14 +18,6 @@ use std::{future::Future, net::IpAddr, sync::Arc, time::Duration};
 /// see no conversation between them.
 pub const DEFAULT_IDLE_TIMEOUT: Duration = Duration::from_secs(60);
 
-/// Default for [`Config::keep_alive_interval`] (20 seconds).
-pub const DEFAULT_KEEP_ALIVE_INTERVAL: Duration = Duration::from_secs(20);
-
-// serde needs a function when specifying a path for `default`
-fn default_keep_alive_interval() -> Option<Duration> {
-    Some(DEFAULT_KEEP_ALIVE_INTERVAL)
-}
-
 /// Default for [`Config::upnp_lease_duration`] (2 minutes).
 pub const DEFAULT_UPNP_LEASE_DURATION: Duration = Duration::from_secs(120);
 
@@ -97,7 +89,7 @@ pub struct CertificateGenerationError(
 );
 
 /// QuicP2p configurations
-#[derive(Clone, Debug, Serialize, Deserialize)]
+#[derive(Clone, Debug, Default, Serialize, Deserialize)]
 pub struct Config {
     /// Specify if port forwarding via UPnP should be done or not. This can be set to false if the network
     /// is run locally on the network loopback or on a local area network.
@@ -127,8 +119,8 @@ pub struct Config {
     ///
     /// Keep-alives prevent otherwise idle connections from timing out.
     ///
-    /// If unspecified, this will default to [`DEFAULT_KEEP_ALIVE_INTERVAL`].
-    #[serde(default = "default_keep_alive_interval")]
+    /// If unspecified, this will default to `None`, disabling keep-alives.
+    #[serde(default)]
     pub keep_alive_interval: Option<Duration>,
 
     /// How long UPnP port mappings will last.
@@ -146,21 +138,6 @@ pub struct Config {
     pub retry_config: RetryConfig,
 }
 
-impl Default for Config {
-    fn default() -> Self {
-        Self {
-            #[cfg(feature = "igd")]
-            forward_port: Default::default(),
-            external_port: Default::default(),
-            external_ip: Default::default(),
-            idle_timeout: Default::default(),
-            keep_alive_interval: default_keep_alive_interval(),
-            upnp_lease_duration: Default::default(),
-            retry_config: Default::default(),
-        }
-    }
-}
-
 /// Retry configurations for establishing connections and sending messages.
 /// Determines the retry behaviour of requests, by setting the back off strategy used.
 #[derive(Clone, Debug, Copy, Serialize, Deserialize)]