message: feat(server): add `Builder::http1_pipeline_flush` configuration
  However, you probably shouldn't use it! It's `doc(hidden)`, since it
  isn't the truest pipeline support. Instead, it just prevents flushing
  until read buffer has been consumed.
  It's only real use is for silly pipeline benchmarks.
diff: @@ -176,6 +176,16 @@ impl<I> Builder<I> {
  self
  }
  + // Sets whether to bunch up HTTP/1 writes until the read buffer is empty.
  + //
  + // This isn't really desirable in most cases, only really being useful in
  + // silly pipeline benchmarks.
  + #[doc(hidden)]
  + pub fn http1_pipeline_flush(mut self, val: bool) -> Self {
  + self.protocol.pipeline_flush(val);
  + self
  + }
  +
  /// Set whether HTTP/1 connections should try to use vectored writes,
  /// or always flatten into a single buffer.
  ///
  