Thread safety

Since people seem to keep trying to use snscrape with threads (despite this not being listed as a feature anywhere) and running into problems (seemingly without searching the issues)...

**snscrape is currently not thread-safe.**

I'd like to evaluate at some point whether it's easy enough to make snscrape thread-safe. One known issue is the Twitter module's guest token manager. Testing thread safety will be an issue, too.

Relevant prior issues: #307 #584 #622

(SEO keywords: threading multithreading)

Start by reviewing prior issues #307, #584, and #622, then inspect the Twitter module's guest token manager, identified as one known thread-safety issue. Determine the scope of making snscrape thread-safe and how thread-safety can be tested; done means the required behavior and test coverage are agreed and implemented.

Written by the indexing model from the issue text.