src: pull AfterConnect from pipe_wrap and tcp_wrap

This commit attempts to address one of the items in
https://github.com/nodejs/node/issues/4641 which is related to
src/pipe_wrap.cc and src/tcp_wrap.cc.

Currently both pipe_wrap.cc and tcp_wrap.cc contain an AfterConnect
function that are almost identical. This commit extracts this function
into ConnectionWrap so that that both can share it.

PR-URL: https://github.com/nodejs/node/pull/8448


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>