R1:f8ed6422f8e2

Provide an auto-refresh mechanism for OAuth providers to deliver fresh tokens

Summary:
Ref T2852. Give OAuth providers a formal method so you can ask them for tokens; they issue a refresh request if necessary.

We could automatically refresh these tokens in daemons as they near expiry to improve performance; refreshes are blocking in-process round trip requests. If we do this for all tokens, it's a lot of requests (say, 20k users * 2 auth mechanisms * 1-hour tokens ~= a million requests a day). We could…