Fix http OutgoingMessage leaks, improve connection reuse a bit

This fixes the leaks on https://github.com/tessel/runtime/issues/655 and also fixes up some halfway-related issues noticed in other posthumous 'http' review.

(Note: there's still one outstanding "leak" warning that I see when running http tests under posix, but not with the superficially associated test's isolate code itself. So I suspect this might actually be related to the global 'tcp-close' but even if not can be dealt with separately should it cause real-world problems.)