Time shouldn't auto-advance unless there's really nothing else going on. In
general that is hard to fix, but this special case is relatively easy.

Fixes: tokio-rs#4614

This uses a destructor, so it will also work if tokio machinery panics while
trying to e.g. spawn a thread.

The auto-advance inhibit count must be decremented after the blocking task is
finished, and crucially after waking the JoinHandle, to prevent the tokio
thread from auto-advancing before tasks awaiting that JoinHandle are
rescheduled.

Specifically, when a blocking task is spawned, but no thread immediately awaits
the JoinHandle, if the main thread parks itself, we previously failed to unpark
it when the blocking task completed. Auto-advance would be re-enabled but could
not happen, a potential deadlock.

It does not always work; and is it happens, the caller has a handle already and
can just pass it in.