src: use libuv's refcounting directly

Don't implement an additional reference counting scheme on top of libuv.
Libuv is the canonical source for that information so use it directly.

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>