Under the JRE, use java.util.concurrent.atomic.LongAdder unconditionally and (except when we need to support GWT/J2CL) directly instead of through our LongAddable interfaces.

Under the JRE, use java.util.concurrent.atomic.LongAdder unconditionally and (except when we need to support GWT/J2CL) directly instead of through our LongAddable interfaces. #4166