ZGC: Steal local stacks instead of flushing them

As part of addressing the issue of excessive mark stack usage, it was observed that sometimes mark stack utilization can be very low. For example, casparcwang@tencent.com reported that when running Apache Zookeeper, mark stack utilization was ~10%.

The problem is that GC workers are a bit too eager to flush mark stacks. Currently, whenever a GC worker has drained its home stripe it will always flush all remaining mark stacks. This causes non-full mark stacks to be flushed. This in turn means that the GC worker who receives that non-full stack (small amount of work) tends to also produce non-full stacks (small amount of work) for other GC workers. This can lead to a vicious cycle, which results in excessive mark stack memory usage.