R1:69ddb0ced631

Issue "anonymous" sessions for logged-out users

Summary:
Ref T4339. Ref T4310. Currently, sessions look like `"afad85d675fda87a4fadd54"`, and are only issued for logged-in users. To support logged-out CSRF and (eventually) external user sessions, I made two small changes:

- First, sessions now have a "kind", which is indicated by a prefix, like `"A/ab987asdcas7dca"`. This mostly allows us to issue session queries more efficiently: we don't have to issue a query at all for anonymous sessions, and can…