RFR (S): 8133530: Add JFR event for evacuation statistics

[Thomas Schatzl]

Hi all,

  can I have reviews for this split-out from 8073013 that adds JFR
support for evacuation statistics.

The code has been changed to (hopefully) correspond better to the way
code that sends JFR events should look like.

CR:
https://bugs.openjdk.java.net/browse/JDK-8133530
Webrev:
http://cr.openjdk.java.net/~tschatzl/8133530/webrev/

Testing:
jprt

Thanks,
  Thomas