Replace Unsafe with VarHandle in java.util.concurrent classes

Relevant classes in java.util.concurrent should be updated to replace the use of Unsafe with VarHandles.

Performance testing needs to ensure there are no performance regressions.

Such classes include ConcurrentHashMap, which is likely used during the boot phase of the JDK, and also used by classes in java.lang.invoke. The VarHandle implementation technique needs to ensure no circular dependencies are introduced.