Current ShenandoahControlThread uses short sleeps to re-evaluate the ever-changing conditions that may trigger the cycle. This is very tedious and problematic when prompt response is needed. It can be done better with timed waits and explicit notifications.

Work in progress:
  https://github.com/openjdk/jdk/compare/master...shipilev:JDK-8264851-shenandoah-control-timedwait