8145092: Use Unified Logging for the GC logging
Summary: JEP-271. VM changes contributed by brutisso, test changes contributed by david.
Reviewed-by: sjohanss, david, brutisso
Contributed-by: bengt.rutisson@oracle.com, david.lindholm@oralce.com

8134953: Make the GC ID available in a central place
Reviewed-by: pliden, jmasa

8136991: [REDO] Additional number of processed references printed with -XX:+PrintReferenceGC after JDK-8047125
Reviewed-by: kbarrett, tschatzl

8079792: GC directory structure cleanup
Reviewed-by: brutisso, stefank, david