[14] RFR 8241984: Shenandoah: enhance GCTimer and JFR support

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241984
  https://hg.openjdk.java.net/jdk/jdk/rev/13e20575efe1

There are were some conflicts during the application, so I had to reapply some hunks myself. Also,
there is no GCTraceTimeWrapper in 14u, only GCTraceTimeImplWrapper.

14u webrev:
  https://cr.openjdk.java.net/~shade/8241984/webrev.14u.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey