Remove any thread-safety arguments to the new messaging patterns

[ptitjes]

It seems that since #146, a lot of emphasis on thread-safety has been put into the specifications (37/ZMTP and the new messaging patterns. I don't understand why as I consider this to be an implementation detail.

I would argue that it is possible to make thread-safe implementations of existing messaging pattern, if, maybe, at a very small cost in performance (using lock-free data-structures).

I would propose to remove all the mentions of thread-safety reasons in all those specifications.