Use lwt_io.is_closed where appropriate

[mseri]

[mseri]

I did it on a branch, the code is uglier and I don't really see the benefit. Is rasing and catching an exception in Lwt so much slower than an if Lwt.is_closed then ... else ...?

[rgrinberg]

Raising and catching an exception is surely much slower than a conditional. But I'm not sure if it's the same change semantically.

if Lwt.is_closed t then foo t

introduces a race because t can be closed before foo actually tries the write.

I suppose the initial check is a decent optimization though.