AbstractNioBoss(Worker)Pool may initialize more than once

Netty version: 3.9.5.Final

Context:
It's not safe to use one volatile variable to prevent the initialization sequence from invoking more than once. You can see that 2 threads can pass the "if(initialized)" check at the same time(refer to the discussion in https://github.com/netty/netty/issues/3249), but not with the atomic CAS.