RFR: Periodic GC

[Aleksey Shipilev]

Following up on yesterday's experiment, here is the Periodic GC implementation:
  http://cr.openjdk.java.net/~shade/shenandoah/periodic-gc/webrev.01/

Only "adaptive" and "dynamic" are enabling it. "aggressive" performs lots of GCs anyway, and
"passive" should not do periodic at all. The default period is 5 minutes, which seems to be a safe
default for detecting really idle applications.

Testing: new test

Thanks,
-Aleksey