Add support for JDK9 Thread.onSpinWait() in spin locks

[jvz]

For example, in BaseMpscLinkedArrayQueue (various places) or as a WaitStrategy, the new Thread.onSpinWait() method from JEP 285 would be useful to add support for. This method adds a JVM hint that it can use a PAUSE instruction in x86 (or equivalent) which is a lower power usage instruction useful for spin locks. Depending on implementation, this may take advantage of multi-release jars as accessing this method reflectively would defeat the purpose of it.

[vyazelenko]

@ivankrylov has created onSpinWaitMultiReleaseJar. From the README file:

This makefile builds a jar that works with and JDK from 6 to 9 (and beyond). Just add this jar to the classpath and insert ThreadHints.onSpinWait() whereever you have a busy loop.

[nitsanw]

The WaitStrategy is implemented by users, who can choose to implement it with onSpinWait if you like, there's nothing for JCTools to implement here.

[franz1981]

@nitsanw I was thinking to introduce https://github.com/real-logic/agrona/blob/master/agrona/src/main/java/org/agrona/hints/ThreadHints.java#L31 to help in cases where spin 4 elements (it happens in many cases, including Dr. Yukov mpsc linked q port and others) right after the empty check, wdyt?