src: setup cluster workers before preloading

We need to process cluster workers before any preload modules is
executed. Otherwise, the child processes are not correctly disovered
as clustered workers inside the preloaded modules.

Fixes: https://github.com/iojs/io.js/issues/1269
PR-URL: https://github.com/iojs/io.js/pull/1314


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>