RFR: Rewire control loop to avoid double cleanup work

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concthread-dbl-cleanup/webrev.01/

Current shortcut cycle does the cleanup twice:

[14.085s][info][gc] GC(2) Pause Init Mark 0.485ms
[14.097s][info][gc] GC(2) Concurrent marking 716M->717M(1024M) 11.723ms
[14.098s][info][gc] GC(2) Pause Final Mark 0.887ms
[14.168s][info][gc] GC(2) Concurrent cleanup 717M->27M(1024M) 69.883ms  <--- (1)
[14.170s][info][gc] GC(2) Concurrent cleanup 27M->27M(1024M) 1.782ms    <--- (2)

We can avoid this by restructuring the code to be more logical.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey