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/client/conn.rs

@@ -82,7 +82,6 @@ where
 #[derive(Clone, Debug)]
 pub struct Builder {
     pub(super) exec: Exec,
-    h1_writev: Option<bool>,
     h1_title_case_headers: bool,
     h1_read_buf_exact_size: Option<usize>,
     h1_max_buf_size: Option<usize>,
@@ -453,7 +452,6 @@ impl Builder {
     pub fn new() -> Builder {
         Builder {
             exec: Exec::Default,
-            h1_writev: None,
             h1_read_buf_exact_size: None,
             h1_title_case_headers: false,
             h1_max_buf_size: None,
@@ -475,11 +473,6 @@ impl Builder {
         self
     }
 
-    pub(super) fn h1_writev(&mut self, enabled: bool) -> &mut Builder {
-        self.h1_writev = Some(enabled);
-        self
-    }
-
     pub(super) fn h1_title_case_headers(&mut self, enabled: bool) -> &mut Builder {
         self.h1_title_case_headers = enabled;
         self
@@ -663,13 +656,6 @@ impl Builder {
                 #[cfg(feature = "http1")]
                 Proto::Http1 => {
                     let mut conn = proto::Conn::new(io);
-                    if let Some(writev) = opts.h1_writev {
-                        if writev {
-                            conn.set_write_strategy_queue();
-                        } else {
-                            conn.set_write_strategy_flatten();
-                        }
-                    }
                     if opts.h1_title_case_headers {
                         conn.set_title_case_headers();
                     }