Fix Krita discarding native threads every couple of seconds

For years Krita has been spawning native threads carelessly. It
happened because of the implementation of QThreadPool::waitForDone(),
which discards threads on every successful call.

We once had a patch for that (9c34fef3),
but it has been reverted due to deadlock regressions it introduced.

This patch adds a bit different implementation of the waiting strategy,
so (I hope) it won't cause any deadlocks.

BUG:367901
BUG:360677