- Update the outdated comments in do_stop_gossiping. It was
storage_service not storage_proxy that used the lock. More
importantly, storage_service does not use it any more.
- Drop the unused timer_callback_lock and timer_callback_unlock API
- Use with_semaphore to make sure the semaphore usage is balanced.
- Add log in gossiper::do_stop_gossiping when it tries to take the
semaphore to help debug hang during the shutdown.
Refs: #4891
Refs: #4971