added max_idle_per_host as an option to the builder (#473)

src/async_impl/client.rs

@@ -67,6 +67,7 @@ struct Config {
     #[cfg(feature = "tls")]
     certs_verification: bool,
     connect_timeout: Option<Duration>,
+    max_idle_per_host: usize,
     #[cfg(feature = "tls")]
     identity: Option<Identity>,
     proxies: Vec<Proxy>,
@@ -100,6 +101,7 @@ impl ClientBuilder {
                 #[cfg(feature = "tls")]
                 certs_verification: true,
                 connect_timeout: None,
+                max_idle_per_host: ::std::usize::MAX,
                 proxies: Vec::new(),
                 redirect_policy: RedirectPolicy::default(),
                 referer: true,
@@ -188,6 +190,8 @@ impl ClientBuilder {
             builder.http2_only(true);
         }
 
+        builder.max_idle_per_host(config.max_idle_per_host);
+
         if config.http1_title_case_headers {
             builder.http1_title_case_headers(true);
         }
@@ -329,6 +333,14 @@ impl ClientBuilder {
         self
     }
 
+    /// Sets the maximum idle connection per host allowed in the pool.
+    //
+    // Default is usize::MAX (no limit).
+    pub fn max_idle_per_host(mut self, max: usize) -> ClientBuilder {
+        self.config.max_idle_per_host = max;
+        self
+    }
+
     /// Only use HTTP/2.
     pub fn h2_prior_knowledge(mut self) -> ClientBuilder {
         self.config.http2_only = true;