Provide a way to move multiple tasks from overflow to regular task queue

In JDK-8152438 we noticed that it can happen that a single thread may work on its own (large) overflow queue while other threads are starved for work. We fixed this there by preferentially moving work from the overflow queue to the task queue. This is done task by task.

This could be improved by filling up the task queue in bulk instead of one-by-one.

Investigate whether it's worth the effort too.