src: make ReqWrap req_ member private

This commit attempts to address one of the items in
https://github.com/nodejs/node/issues/4641 which is
related to src/req-wrap.h and making the req_ member private.

PR-URL: https://github.com/nodejs/node/pull/8532


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>