Initialization has completed and all future calls should finish immediately.
No initialization has run yet, and no thread is currently using the Once.
Some thread has previously attempted to initialize the Once, but it panicked,
so the Once is now poisoned. There are no other threads currently accessing
this Once.
Some thread is currently attempting to run initialization and there are threads
waiting for it to finish.
Some thread is currently attempting to run initialization. It may succeed,
so all future threads need to wait for it to finish.