zlib: do not emit event on *Sync() methods

Asynchronous functions in `zlib` should not emit the close event.

This fixes an issue where asynchronous calls in a for loop could exhaust
memory because the pending event prevents the objects from being garbage
collected.

Fixes: https://github.com/nodejs/node/issues/1668
PR-URL: https://github.com/nodejs/node/pull/5707


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>