RFR: 8236851: Shenandoah: More details in Traversal GC event messages

[Roman Kennke]

We currently only print e.g. "Pause Init Traversal" in event messages
for Traversal GC. We should also include information whether or not the
cycle also does unload classes and/or process references, like we do for
the normal mode.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8236851

Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8236851/webrev.00/

Testing: hotspot_gc_shenandoah, manual inspection of hs_err files

Can I please get a review?

Thanks,
Roman