feat(http2): add adaptive window size support using BDP (#2138)

This adds support for calculating the Bandwidth-delay product when using
HTTP2. When a DATA frame is received, a PING is sent to the remote.
While the PING acknoledgement is outstanding, the amount of bytes of all
received DATA frames is accumulated. Once we receive the PING
acknowledgement, we calculate the BDP based on the number of received
bytes and the round-trip-time of the PING. If we are near the current
maximum window size, the size is doubled.

It's disabled by default until tested more extensively.

src/client/mod.rs

@@ -996,6 +996,16 @@ impl Builder {
         self
     }
 
+    /// Sets whether to use an adaptive flow control.
+    ///
+    /// Enabling this will override the limits set in
+    /// `http2_initial_stream_window_size` and
+    /// `http2_initial_connection_window_size`.
+    pub fn http2_adaptive_window(&mut self, enabled: bool) -> &mut Self {
+        self.conn_builder.http2_adaptive_window(enabled);
+        self
+    }
+
     /// Sets the maximum idle connection per host allowed in the pool.
     ///
     /// Default is `usize::MAX` (no limit).