Router that ensures the same DB is used within a request

[gnarvaja]

Uses thread local storage (o gevent local storage if available) to store the current DB in use. In the first call for a request, it sets the current db in use and returns always the same.

At the start of each request, the middleware cleans the current DB.

[jsocol]

Super late to this, but I think this has two great ideas in it:

1. supporting gevent greenlet locals
2. stick to the same DB (which I think would fix ValueError: Can't do subqueries with queries on different DBs. #20)

I want to break these apart because I think there's some nuance to (1) (e.g. just because gevent is installed doesn't mean it's in use) and for (2) I don't think we need a different router—I think it's arguably correct behavior. So I think we can address these separately and move them both forward.

[jsocol]

(Actually, re-reading some old issues/code, I feel like Gevent patches threading.local by default? I'd rather confirm that there's an issue before writing code to fix it.)