lib,test: remove publicly exposed freelist

The freelist module was deprecated in io.js and moved to an
internal module. This commit removes public access to freelist,
while leaving the internal module, which is still in use.

Fixes: https://github.com/nodejs/node/issues/569
PR-URL: https://github.com/nodejs/node/pull/3738


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>