lep: request all the things

I think I disagree. We should not define the "meaning" of omitting a callback arbirtrarily. Instead I'd prefer to use a generic pattern.

The uv_fs_xx class of functions also allows you to omit the callback. There it means that the operation should be blocking.

uv_close() allows one to omit the callback. There it means that the operation is still asynchronous but no callback gets called.

My preference is to say that passing no callback to an async function means "blocking".

In addition, what's weird is that in the case of uv_write() the semantic of the function also changes subtly. Whereas the async version of uv_write() would always write the entire buffer, the "try" variant would only write as much bytes as it could.