Feature/node ipc channel

IPC channel for parents to make requests of their children... and vice versa.

Preparation for strong-pm to communicate with strong-supervisor using node IPC.


I meant return the instance that is already attached to cluster if there already is one instead of throwing. My point was to write the `return cluster._strongControlChannel = cluster._strongControlChannel || createServer();` so the user doesn't have to do so themselves without the advantage of "knowing" to look for `cluster._strongControlChannel`. The implementation would need changes to account for that and possibly allow replacing `onRequest`. That may be more work than it is worth.