util.dbuffer: Add efficient shortcuts for discard() in certain cases

If the buffer is already empty, nothing to do. If we're throwing away the
whole buffer, we can just empty it and avoid read_chunk() (which in turn
may collapse()). These shortcuts are much more efficient.

util/dbuffer.lua

@@ -91,8 +91,12 @@ function dbuffer_methods:read_until(char)
 end
 
 function dbuffer_methods:discard(requested_bytes)
-	if requested_bytes > self._length then
-		return nil;
+	if self._length == 0 then return true; end
+	if not requested_bytes or requested_bytes >= self._length then
+		self.front_consumed = 0;
+		self._length = 0;
+		for _ in self.items:consume() do end
+		return true;
 	end
 
 	local chunk, read_bytes = self:read_chunk(requested_bytes);