RFR: 8241666: Enhance log messages in ReferenceProcessor

[Stefan Johansson]

Hi,

Please review this change to improve some logging messages.

Webrev: http://cr.openjdk.java.net/~sjohanss/8241666/00/
Issue: https://bugs.openjdk.java.net/browse/JDK-8241666

Summary
The old logging messages were a bit unclear and when checking if the 
tests needed to be updated I found some unused static members that I 
removed.

Testing
Locally run GC tests and mach5 tier1 and tier2.

Thanks,
Stefan