Only send MAX_STREAMS when >1/8 of flow control window is consumed

[Ralith]

Minor optimization to reduce outgoing traffic when receiving large numbers of streams. Mirrors existing logic for data flow control, e.g.

quinn/quinn-proto/src/connection/streams/state.rs

Lines 849 to 855 in 7b97a3f

	// Only announce a window update if it's significant enough
	// to make it worthwhile sending a MAX_DATA frame.
	// We use a fraction of the configured connection receive window to make
	// the decision, to accommodate for connection using bigger windows requiring
	// less updates.
	let diff = self.local_max_data - self.sent_max_data.into_inner();
	ShouldTransmit(diff >= (self.receive_window / 8))

[djc]

Nice!