Making LocaleBorg thread-safe.

[felixfontein]

Fixes #2046 as discussed there.

[felixfontein]

The failed test is due to a gist not being loaded. I've seen this error several times appearing and disappearing over the last hours (for different branches), so I assume it's not my fault ;)

[felixfontein]

I did some more testing, it seems to work fine. I also did some timing tests with one of my smaller blogs; the new code with a thread local variable and a lock didn't result in a noticeable performance drop (11.1 vs. 11.2 seconds for master vs threadsafe-localeborg in single-threaded mode), and this performance drop was the same when I replaced the Lock() object with an object with empty __enter__ and __exit__ methods and the thread-local storage with a simple shared state; so it is probably more the fault of formatted_date calling __set_locale and/or current_lang calling __get_shared_state instead of directly accessing it.

[ralsina]

LGTM let's merge it and see.

[felixfontein]

Thanks for merging!