feat(server): Make the server code an optional feature (#2334)

cc #2223 

BREAKING CHANGE: The HTTP server code is now an optional feature. To
  enable the server, add `features = ["server"]` to the dependency in
  your `Cargo.toml`.

src/headers.rs

@@ -30,6 +30,7 @@ fn connection_has(value: &HeaderValue, needle: &str) -> bool {
 }
 
 #[cfg(feature = "http1")]
+#[cfg(feature = "server")]
 pub fn content_length_parse(value: &HeaderValue) -> Option<u64> {
     value.to_str().ok().and_then(|s| s.parse().ok())
 }