Rule for reference processing time

Now (soon) that there is an event for the reference processing time, we may want to consider a rule that makes use of it. It can be a bit tricky, and we may want to make it depend on the type of GC that is in use. Perhaps it will, for the GCs where we have something with a high degree of precision to say, be too similar to the safepointing rule. Anyways, food for thought. 

8292266: Add JFR event for total Reference processing time by sangheon · Pull Request #11230 · openjdk/jdk (github.com)