cluster: refactor module into multiple files

This commit splits the existing cluster module into several
internal modules. More specifically, the cluster master and
worker implementations are separated, and the various data
structures are separated.

PR-URL: https://github.com/nodejs/node/pull/10746


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>