src: rename handle__ to handle_ in HandleWrap

Commit 46633934 (src: pull
OnConnection from pipe_wrap and tcp_wrap) removed the private handle_
member from TCPWrap which should allow us to rename the private
handle__ member in HandleWrap.

PR-URL: https://github.com/nodejs/node/pull/8712


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>