RFR (S) 8231932: Shenandoah: conc/par GC threads ergonomics overrides user settings

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8231932

Fix:
  https://cr.openjdk.java.net/~shade/8231932/webrev.01/

This manifests in tier1 tests, and that is actually the UX problem. New test captures it directly.
Patched code favors adjusting the setting that was selected ergonomically, which leaves the user
setting alone.

Also, it is awkward to adjust the GC threads settings silently (which is why test failed without
proper message), and we should fail on misconfiguration right away, which explains the adjustments
in existing tests.

Testing: new test, hotspot_gc (with Shenandoah), tier1 (with Shenandoah), hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey