Add support for platforms that do not have sun.misc.Unsafe

[alexandru]

We've got problems with Android at least and possibly with the upcoming Java 9. See related issue #269.

1. the Atomic classes should support an Unsafe-free version, based on "atomic field updaters", supported since Java 5, see for example AtomicIntegerFieldUpdate as sample
2. we need to either fork the required JCTools implementations or find a replacement to be used for platforms that don't support Unsafe

[nitsanw]

You can use the org.jctools.queues.atomic.* queues when Unsafe is not available.
From #269 it would seem this can also be fixed with a patch to UnsafeAccess.

[alexandru]

@nitsanw issue #269 can be fixed with a patch to JCTools' UnsafeAccess, I was preparing it.

[alexandru]

@nitsanw see I've created a pull request, let me know what you think: JCTools/JCTools#141

/cc @taig