RFR (S) 8237586: Shenandoah: provide option to disable periodic GC

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8237586

Webrev:
  https://cr.openjdk.java.net/~shade/8237586/webrev.01/

The VM option is unsigned, which leaves as with "0" as special value. It also matches the behavior
of G1PeriodicGCInterval and GuaranteedSafepointInterval.

Testing: hotspot_gc_shenandoah (includes new testcases)

-- 
Thanks,
-Aleksey