load-namespaces not parallel-safe

[SevereOverfl0w]

I am using integrant with eftest, and starting systems in my fixtures. Because load-namespaces is not thread safe, this causes sporadic namespace errors with eftest.

I can sync on my side, but I thought it might be worth doing some kind of sync in integrant.

[weavejester]

Loading in Clojure is pretty much all thread unsafe, as far as I know. It might be worth adding something to the docstring, but I don't think we should add in a sync by default.