RFR: Pacing improvements

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/pacing-exponential/webrev.01/

Contains two changes:

 a) Exponential backoff for pacing waits: there is no need to sleep for a millisecond on larger
pacing delays, and the sleep would be more than 1 ms anyway due to scheduling constraints. This
allows to crank up ShenandoahPacingMaxDelay to a very high value to emulate bulk allocation stalls.

 b) More verbose pacing histogram: prints sum per histogram level, and complete summary

Testing: tier3_gc_shenandoah, eyeballing gc+stats

Thanks,
-Aleksey