do not destroy free (i.e. intentionally idle) sockets (node 0.11.x "actually useful keep-alive" behavior broken in https://github.com/tessel/runtime/commit/7e42f6fb8402ee50d612f5b889b5192ef68ebf15)
we do not want outgoing message to end socket on its own (close is managed elsewhere, so connection may be reused by keep-alive!)
make sure outbox ends, then unpipe outbox from socket when it does. this gets rid of the EventEmitter leaks on #655, although still seeing one warning during http tests.
Merge #673 r=@jiahuang: 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.)