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/common/io/mod.rs

@@ -1,3 +1,4 @@
 mod rewind;
 
 pub(crate) use self::rewind::Rewind;
+pub(crate) const MAX_WRITEV_BUFS: usize = 64;

src/common/io/rewind.rs

@@ -84,6 +84,14 @@ where
         Pin::new(&mut self.inner).poll_write(cx, buf)
     }
 
+    fn poll_write_vectored(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        bufs: &[io::IoSlice<'_>],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_write_vectored(cx, bufs)
+    }
+
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_flush(cx)
     }
@@ -91,6 +99,10 @@ where
     fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_shutdown(cx)
     }
+
+    fn is_write_vectored(&self) -> bool {
+        self.inner.is_write_vectored()
+    }
 }
 
 #[cfg(test)]