Replace volatile usage with VarHandle release/acquire for better performance

[hmottestad]

We typically use volatile for variables that can be updated and read by multiple threads. Unless we need total order for read/write to multiple variables then it is may be safe to use release/acquire (RA.

See:

• https://gee.cs.oswego.edu/dl/html/j9mm.html#volatilesec
• https://gee.cs.oswego.edu/dl/html/j9mm.html#rasec