[14] RFR (S) 8244737: Shenandoah: move mode code to gc/shenandoah/mode

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8244737
  https://hg.openjdk.java.net/jdk/jdk/rev/a8dd9e32110b

Again, the context is a bit different, so I re-made the actual moves, and applied the rest of the
hunks automatically. 14u webrev:
  https://cr.openjdk.java.net/~shade/8244737/webrev.14u.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey