Extract worker logic out of ProcessEnvironment #913 (#939)

This is part of the effort to unblock #913. Two reasons for this change

1. `worker` definitely does not rely on `ProcessEnvironment` at all, and therefore it is cleaner to make it independent of `ProcessEnvironment`.
2. If it stays as a member method of `ProcessEnvironment`, `multiprocess.Process` will get stuck on `start()` if the parent process is also a `multiprocess.Process`, for unknown reason though (tried investigation but haven't figured out).