better orphaned worker detection

[eklitzke]

The current code for this seems to work well on Linux but is racy. A more elegant and reliable solution can be implemented using pipes.

[eklitzke]

Just so I remember, the idea is that the workers open a pipe to their parent. If the parent (i.e. intermediate zygote) exits, the children will receive a close event on the pipe, which signals that the parent is gone and the worker has been re-parented to init.