feat(server): add experimental pipeline flush aggregation option to Http

By enabling `Http::pipeline`, the connection will aggregate response
writes to try to improve sending more responses in a single syscall.

src/http/conn.rs

@@ -47,6 +47,9 @@ where I: AsyncRead + AsyncWrite,
         }
     }
 
+    pub fn set_flush_pipeline(&mut self, enabled: bool) {
+        self.io.set_flush_pipeline(enabled);
+    }
 
     fn poll2(&mut self) -> Poll<Option<Frame<http::MessageHead<T::Incoming>, http::Chunk, ::Error>>, io::Error> {
         trace!("Conn::poll()");

src/http/io.rs

@@ -13,6 +13,7 @@ const INIT_BUFFER_SIZE: usize = 8192;
 pub const MAX_BUFFER_SIZE: usize = 8192 + 4096 * 100;
 
 pub struct Buffered<T> {
+    flush_pipeline: bool,
     io: T,
     read_blocked: bool,
     read_buf: BytesMut,
@@ -31,6 +32,7 @@ impl<T> fmt::Debug for Buffered<T> {
 impl<T: AsyncRead + AsyncWrite> Buffered<T> {
     pub fn new(io: T) -> Buffered<T> {
         Buffered {
+            flush_pipeline: false,
             io: io,
             read_buf: BytesMut::with_capacity(0),
             write_buf: WriteBuf::new(),
@@ -38,6 +40,10 @@ impl<T: AsyncRead + AsyncWrite> Buffered<T> {
         }
     }
 
+    pub fn set_flush_pipeline(&mut self, enabled: bool) {
+        self.flush_pipeline = enabled;
+    }
+
     pub fn read_buf(&self) -> &[u8] {
         self.read_buf.as_ref()
     }
@@ -139,7 +145,9 @@ impl<T: Write> Write for Buffered<T> {
     }
 
     fn flush(&mut self) -> io::Result<()> {
-        if self.write_buf.remaining() == 0 {
+        if self.flush_pipeline && self.read_buf.is_empty() {
+            Ok(())
+        } else if self.write_buf.remaining() == 0 {
             self.io.flush()
         } else {
             loop {