Libprocess: Implement process::Clock::finalize

Tracks this TODO.

The Clock is initialized with a callback that, among other things, will dereference the global process_manager object.

When libprocess is shutting down, the process_manager is cleaned up. Between cleanup and termination of libprocess, there is some chance that a Timer will time out and result in dereferencing process_manager.

Proposal

• Implement Clock::finalize. This would clear:
  • existing timers
  • process-specific clocks
  • ticks
• Change process::finalize.
  1. Resume the clock. (The clock is only paused during some tests.) When the clock is not paused, the callback does not dereference process_manager.
  2. Clean up process_manager. This terminates all the processes that would potentially interact with Clock.
  3. Call Clock::finalize.