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/tcp.rs

@@ -293,6 +293,15 @@ mod addr_stream {
             self.project().inner.poll_write(cx, buf)
         }
 
+        #[inline]
+        fn poll_write_vectored(
+            self: Pin<&mut Self>,
+            cx: &mut task::Context<'_>,
+            bufs: &[io::IoSlice<'_>],
+        ) -> Poll<io::Result<usize>> {
+            self.project().inner.poll_write_vectored(cx, bufs)
+        }
+
         #[inline]
         fn poll_flush(self: Pin<&mut Self>, _cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
             // TCP flush is a noop
@@ -303,6 +312,15 @@ mod addr_stream {
         fn poll_shutdown(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
             self.project().inner.poll_shutdown(cx)
         }
+
+        #[inline]
+        fn is_write_vectored(&self) -> bool {
+            // Note that since `self.inner` is a `TcpStream`, this could
+            // *probably* be hard-coded to return `true`...but it seems more
+            // correct to ask it anyway (maybe we're on some platform without
+            // scatter-gather IO?)
+            self.inner.is_write_vectored()
+        }
     }
 
     #[cfg(unix)]