RFR (S) 8247367: Shenandoah: pacer should wait on lock instead of exponential backoff

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8247367

After JDK-8247310, we can just use the wait/notify on the newly introduced lock to coordinate
wakeups of paced threads. This avoids doing exponential backoff that introduces additional latency.

Fix:
  https://cr.openjdk.java.net/~shade/8247367/webrev.01/

Testing: hotspot_gc_shenandoah; benchmarks

-- 
Thanks,
-Aleksey