Lwt 4.0.1

Bugs fixed

• Race condition in worker thread management in Lwt_unix (#569, diagnosed Gabe Levi).
• Hang in Lwt_unix.read on Windows (#574, #569, 86a6baf, diagnosed Gabe Levi).
• Docs: note that Lwt_io.open_file for writing truncates the file by default (#570, reported Tóth Róbert).