Add IPC channel disconnect method and process isolate method.

[AndreasMadsen]

This allow the following issues to be fixed:

• issue process still alive, when fork channel closed #2344
• issue Child processes never exit #1740
• issue child_process.fork() - parent process signals are relayed to child process #2292
• issue child process use child_process.fork never stop #1363
• issue How to end cluster's worker process gracefully? #2088

disconnect:
This allow one to disconnect the IPC between parent and child, allowing the child to die gracefull.

isolate:
This allow the parent to die gracefull independent of its child. This makes it easy to pull a process in the background should you wich so.

@ry @bnoordhuis
I have a felling that the process._channel.close() is async but do not have a callback, could you confirm or disconfirm this.

This also solves some future issues related to cluster 2.0 (see #2038)

[AndreasMadsen]

Disconnect has landed in 836344c.
Isolate is renamed to independent and is moved to #2676.