RFR (M): 8008918: Reference statistics events for the tracing framework

[Erik Helin]

Hi all,

this change adds the vm/gc/reference/statistics event to all collectors.

Please note that this change depends on "8009232: Improve stats 
gathering code for reference processor" which is also out for review on
hotspot-gc-dev at openjdk.java.net.

Webrev:
http://cr.openjdk.java.net/~ehelin/8008918/webrev.00/

Bug:
https://jbs.oracle.com/bugs/browse/JDK-8008918

Testing:
JPRT

Thanks,
Erik