tpool : fix calculation of currently active tasks in maybe_wake_or_create_thread()

A task that is executed,could be counted as waiting (after wait_begin()
before wait_end()) or as long-running (callback runs for a long time).

If task is both marked waiting and long-running, then calculation of
current concurrency (# of executing tasks - long tasks - waiting tasks)
would be wrong, as single task is counted twice.

the number above could go negative, with unsigned arithmetic it will become a huge.

As a result, maybe_wake_or_create_thread() would neither wake nor create
a thread, when it should. Which may result in a deadlock.