RFR (S) 8249649: Shenandoah: provide per-cycle pacing stats

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8249649

Current pacer statistics is printing only at the end of the run. It would be convenient to have it
in per-cycle statistics too.

Webrev:
  https://cr.openjdk.java.net/~shade/8249649/webrev.01/

Testing: hotspot_gc_shenandoah; tier{1,2} with Shenandoah; eyeballing gc logs

-- 
Thanks,
-Aleksey