Making fastjet 3.1.0 more thread-safe

Here are a number of commits to make fastjet 3.1.0 more thread safe. 
I think you should not use std::atomic at top level here. The object is big and it forces the compiler to use an implementation of std::atomic which is found only in libatomic. I'd rather avoid depending on it. Can you rework it so that you do not need such a wide scope atomic?