This patch splits the handling of `isMainThread` and
`ownsProcessState` from conditionals in
`lib/internal/bootstrap/node.js` into different scripts under
`lib/internal/bootstrap/switches/`, and call them accordingly
from C++ after `node.js` is run.

This:

- Creates a common denominator of the main thread and the worker
  thread bootstrap that can be snapshotted and shared by
  both.
- Makes it possible to override the configurations on-the-fly.

…ions