[8] RFR: Parallel cleaning improvements

[Aleksey Shipilev]

In sh/jdk8, we have gc/shared/parallelCleaning.* that is only used by Shenandoah. We did this a long
time ago thinking we would share this with G1. But, in later releases, we have decided to have
gc/shenandoah/shenandoahParallelCleaning.* instead. sh/jdk11 has it, jdk/jdk has it. sh/jdk8 should
do the same.

Additionally, there are format specifier problems in ~ShenandoahStringSymbolTableUnlinkTask, which
prevent it to build with C++11 enabled. Those are fixed in a separate changeset.

Webrev:
  https://cr.openjdk.java.net/~shade/shenandoah/8u-parallel-cleaning/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug, release}

-- 
Thanks,
-Aleksey