Shenandoah: improve parallelism for concurrent thread root scans

Following JDK-8256298, there are a few minor performance issues with the implementation.

First, in the spirit of JDK-8246100, we should be scanning the Java threads the last, as they have the most parallelism. Less parallel, or lightweight roots should be scanned before them to improve overall parallelism.

Second, claiming each thread dominates the per-thread processing cost. We should really be doing chunked processing.