async_wrap: ensure all objects have internal field

If the constructor can't assign a class id then the heap snapshot will
not be able to report the object. So ensure that all AsyncWrap instances
use a FunctionTemplate instance with an internal field count >= 1.

PR-URL: https://github.com/nodejs/node/pull/3139


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-by: Stephen Belanger <admin@stephenbelanger.com>