RFR (M): 8209062: Clean up G1MonitoringSupport

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that cleans up the
G1Monitoringsupport class to make future extensions easier?

The changes are of the usual kind for such old code:
 - reduce getters by passing some information in constructors
 - avoid using trivial getters
 - remove useless and trivial getters
 - remove overridden getters that return the same results as the
original ones
 - rename member variables to clearly state whether they are about
generations or spaces
 - clean up obsolete code/comments

CR:
https://bugs.openjdk.java.net/browse/JDK-8209062
Webrev:
http://cr.openjdk.java.net/~tschatzl/8209062/webrev/
Testing:
hs-tier1-4, jdk-tier1-3

Depends on JDK-8209061 also out for review.

Thanks,
  Thomas