Concurrent thread-stack processing in the Z Garbage Collector

The Z Garbage Collector (ZGC) goal was to not exceed 10 ms pause times. Once we got there, we wanted to be more ambitious and make pauses sub-millisecond. To do that, you can no longer scan thread stacks inside of GC pauses. This talk is about "JEP 376: ZGC: Concurrent thread-stack processing", where we moved this processing out to concurrent phases instead, to achieve the low latency GC dream.