Support for delayed shutdown of overflow workers

[hauleth]

Sometimes creating new worker can be expensive, so it would be useful to be able to delay shutdown of the overflow workers to stay for some time after it became idle just in case if there will be another incoming request during that time. This is a quite common pattern that work come in waves, so we can often assume, that if there is overflow, then it may be there for a little while, so stopping and starting workers continuously may not be the best solution.