Summary:
SafepointStatistics is a periodic event, both events are experimental
now and disbaled by default

Test Plan:
jdk/jfr/event/runtime/TestSafepointEvents.java
jdk/jfr/event/compiler/TestHugeObjectAllocationSample.java
jdk/jfr/event/metadata/TestLookForUntestedEvents.java

Reviewed-by: kuaiwei

Issue: https://github.com/alibaba/dragonwell8/issues/116