RFR(S): 8245447: Add JFR SafepointStatistics Event

[Denghui Dong]

Hi team,

Could you review this small patch that adds SafepointStatistics Event,
I think it could help users quickly evaluate the impact of safepoint on Applications(Not sure if this is a general requirement). 

And in the current patch, this event depends on PerfData, so users can't read right value if they disable PerfData by -XX:-PerfData

Bug: https://bugs.openjdk.java.net/browse/JDK-8245447
Webrev: http://cr.openjdk.java.net/~ddong/8245447/webrev/

Thanks,
Denghui