lib: simplify nextTick() usage

This commit removes unnecessary nextTick() closures and adds some
shared nextTick() callbacks for better re-use.

PR-URL: https://github.com/nodejs/io.js/pull/1612


Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>