RFR [8u]: Prettify Shenandoah JDK 8 logging

[Aleksey Shipilev]

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/8u-logging-pretty/webrev.01/

Wanted to do this for a while now. Our 8u logging uses our own translation layer to use Unified
Logging -like interface in Shenandoah code. However, that translation code produces weirdly
formatted result, which complicates parsing/following GC logs that our adopters report. We can
significantly improve that! It requires fiddling with the mess that C varargs are, but the
improvement is worth it.

Logs before/after:
  http://cr.openjdk.java.net/~shade/shenandoah/8u-logging-pretty/before.txt
  http://cr.openjdk.java.net/~shade/shenandoah/8u-logging-pretty/after.txt

Testing: hotspot_gc_shenandoah; eyeballing GC logs with different logging options

-- 
Thanks,
-Aleksey