feat(all): add socket timeouts

Methods added to `Client` and `Server` to control read and write
timeouts of the underlying socket.

Keep-Alive is re-enabled by default on the server, with a default
timeout of 5 seconds.

BREAKING CHANGE: This adds 2 required methods to the `NetworkStream`
  trait, `set_read_timeout` and `set_write_timeout`. Any local
  implementations will need to add them.

src/client/request.rs

@@ -2,7 +2,6 @@
 use std::marker::PhantomData;
 use std::io::{self, Write};
 
-#[cfg(feature = "timeouts")]
 use std::time::Duration;
 
 use url::Url;
@@ -44,14 +43,12 @@ impl<W> Request<W> {
     pub fn method(&self) -> method::Method { self.method.clone() }
 
     /// Set the write timeout.
-    #[cfg(feature = "timeouts")]
     #[inline]
     pub fn set_write_timeout(&self, dur: Option<Duration>) -> io::Result<()> {
         self.message.set_write_timeout(dur)
     }
 
     /// Set the read timeout.
-    #[cfg(feature = "timeouts")]
     #[inline]
     pub fn set_read_timeout(&self, dur: Option<Duration>) -> io::Result<()> {
         self.message.set_read_timeout(dur)