src: simplify handlewrap state tracking logic

This also updates the tests to expect that a closed handle has no
reference count.

PR-URL: https://github.com/nodejs/node/pull/6395


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>