Raise an assertion error when a Runtime is used by client/worker creation/usage.

What was changed

Why?

Checklist

1. Closes [Feature Request] Better prevention of accidental Runtime use over forks #1201

2. How was this tested:

• Unit tests were added that create a fork and expect an assertion error for the following:
  • creating a client with Client.connect after Runtime.default was called pre-fork
  • using a client created pre-fork
  • creating a worker after Runtime.default was called pre-fork
  • using a worker created pre-fork

3. Any docs updates needed?