Add Pool.onQuiescent(Cell)

[JanKoelzer]

pool.onQuiescent(cell) {v => …}

Or maybe use Cell.getResult(): Future[V]? This could return if putFinal has been called but pool is still running.

[JanKoelzer]

If the first alternative is used, getResult() could be private.

[JanKoelzer]

I have proposed a fix that adds pool.onQuiescent(cell)(callback). But this might interact unpredictably with quiescentResolveCycles, quiescentResolveCell and quiescentResolveDefaults.

That could be a reason to use Cell.getResult(): Future[V]. But: There might be cases, where the future does not get resolved while pool.onQuiescent(cell) does.