use a ring buffer for the datagram queue (#4223)

internal/utils/ringbuffer/ringbuffer.go

@@ -8,7 +8,7 @@ type RingBuffer[T any] struct {
 	full             bool
 }
 
-// Init preallocs a buffer with a certain size.
+// Init preallocates a buffer with a certain size.
 func (r *RingBuffer[T]) Init(size int) {
 	r.ring = make([]T, size)
 }
@@ -62,6 +62,16 @@ func (r *RingBuffer[T]) PopFront() T {
 	return t
 }
 
+// PeekFront returns the next element.
+// It must not be called when the buffer is empty, that means that
+// callers might need to check if there are elements in the buffer first.
+func (r *RingBuffer[T]) PeekFront() T {
+	if r.Empty() {
+		panic("github.com/quic-go/quic-go/internal/utils/ringbuffer: peek from an empty queue")
+	}
+	return r.ring[r.headPos]
+}
+
 // Grow the maximum size of the queue.
 // This method assume the queue is full.
 func (r *RingBuffer[T]) grow() {