- Moves fork handling from behind the event_engine_fork experiment
  - No experiment (parity with original fork implementation):
    - Thread pool and timer manager will go through the fork as expected
    - Epoll poller fd will be recreated in the child
    - EventHandles are closed in the child process.
  - Behind experiment (new functionality):
    - File descriptors are tracked in the collection
    - FD generation is tracked and checked on every Posix call.
    - AresResolver is reset on fork
- Fixes to address race between PosixEngine destructor and fork
  handling.
  - Thread pool will properly handle fork even if the parent PosixEngine
    instance entered the destructor before the fork call
  - Thread pool fork handler will wait for the parent event engine
    destructor to finish before shutting down threads.