Async upgrade in cork

@aPoCoMiLogin and @MeredithRodneyMcKay submitted a fix for the buggy case of upgrading a HttpSocket into WebSocket inside cork() callback from inside an "async" callback (such as a timer or anything non-uWS).

People had reported various workarounds for this bug - those are no longer necessary.

beginWrite

@notaphplover has added beginWrite() for when you need to establish Transfer-Encoding: Chunked writing path before you know what to write. Calling beginWrite() locks you into the write() chunked writing path immediately.