feat(http2): add http2_max_send_buf_size option to client and server

This value is like a high-water mark. It applies per stream. Once a
stream has buffered that amount of bytes to send, it won't poll more
data from the `HttpBody` until the stream has been able to flush under
it.

src/proto/h2/client.rs

@@ -36,6 +36,7 @@ type ConnEof = oneshot::Receiver<Never>;
 const DEFAULT_CONN_WINDOW: u32 = 1024 * 1024 * 5; // 5mb
 const DEFAULT_STREAM_WINDOW: u32 = 1024 * 1024 * 2; // 2mb
 const DEFAULT_MAX_FRAME_SIZE: u32 = 1024 * 16; // 16kb
+const DEFAULT_MAX_SEND_BUF_SIZE: usize = 1024 * 1024; // 1mb
 
 #[derive(Clone, Debug)]
 pub(crate) struct Config {
@@ -50,6 +51,7 @@ pub(crate) struct Config {
     #[cfg(feature = "runtime")]
     pub(crate) keep_alive_while_idle: bool,
     pub(crate) max_concurrent_reset_streams: Option<usize>,
+    pub(crate) max_send_buffer_size: usize,
 }
 
 impl Default for Config {
@@ -66,6 +68,7 @@ impl Default for Config {
             #[cfg(feature = "runtime")]
             keep_alive_while_idle: false,
             max_concurrent_reset_streams: None,
+            max_send_buffer_size: DEFAULT_MAX_SEND_BUF_SIZE,
         }
     }
 }
@@ -76,6 +79,7 @@ fn new_builder(config: &Config) -> Builder {
         .initial_window_size(config.initial_stream_window_size)
         .initial_connection_window_size(config.initial_conn_window_size)
         .max_frame_size(config.max_frame_size)
+        .max_send_buffer_size(config.max_send_buffer_size)
         .enable_push(false);
     if let Some(max) = config.max_concurrent_reset_streams {
         builder.max_concurrent_reset_streams(max);