perf(lib): re-enable writev support (#2338)

Tokio's `AsyncWrite` trait once again has support for vectored writes in
Tokio 0.3.4 (see tokio-rs/tokio#3149).

This branch re-enables vectored writes in Hyper for HTTP/1. Using
vectored writes in HTTP/2 will require an upstream change in the `h2`
crate as well.

I've removed the adaptive write buffer implementation
that attempts to detect whether vectored IO is or is not available,
since the Tokio 0.3.4 `AsyncWrite` trait exposes this directly via the
`is_write_vectored` method. Now, we just ask the IO whether or not it
supports vectored writes, and configure the buffer accordingly. This
makes the implementation somewhat simpler.

This also removes `http1_writev()` methods from the builders. These are
no longer necessary, as Hyper can now determine whether or not
to use vectored writes based on `is_write_vectored`, rather than trying
to auto-detect it.

Closes #2320 

BREAKING CHANGE: Removed `http1_writev` methods from `client::Builder`,
  `client::conn::Builder`, `server::Builder`, and `server::conn::Builder`.
  
  Vectored writes are now enabled based on whether the `AsyncWrite`
  implementation in use supports them, rather than though adaptive
  detection. To explicitly disable vectored writes, users may wrap the IO
  in a newtype that implements `AsyncRead` and `AsyncWrite` and returns
  `false` from its `AsyncWrite::is_write_vectored` method.

src/server/mod.rs

@@ -284,27 +284,6 @@ impl<I, E> Builder<I, E> {
         self
     }
 
-    /// Set whether HTTP/1 connections should try to use vectored writes,
-    /// or always flatten into a single buffer.
-    ///
-    /// # Note
-    ///
-    /// Setting this to `false` may mean more copies of body data,
-    /// but may also improve performance when an IO transport doesn't
-    /// support vectored writes well, such as most TLS implementations.
-    ///
-    /// Setting this to true will force hyper to use queued strategy
-    /// which may eliminate unnecessary cloning on some TLS backends
-    ///
-    /// Default is `auto`. In this mode hyper will try to guess which
-    /// mode to use.
-    #[cfg(feature = "http1")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "http1")))]
-    pub fn http1_writev(mut self, val: bool) -> Self {
-        self.protocol.http1_writev(val);
-        self
-    }
-
     /// Sets whether HTTP/1 is required.
     ///
     /// Default is `false`.