src: pull OnConnection from pipe_wrap and tcp_wrap

One of the issues in #4641 concerns OnConnection in pipe_wrap and
tcp_wrap which are very similar with some minor difference in how
they are coded. This commit extracts OnConnection so both these
classes can share the same implementation.

PR-URL: https://github.com/nodejs/node/pull/7547


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>