R1:c58506aeaace

Give sessions real PHIDs and slightly modernize session queries

Summary:
Ref T13222. See PHI873. I'm preparing to introduce a new MFA "Challenge" table which stores state about challenges we've issued (to bind challenges to sessions and prevent most challenge reuse).

This table will reference sessions (since each challenge will be bound to a particular session) but sessions currently don't have PHIDs. Give them PHIDs and slightly modernize some related code.

Test Plan:
- Ran migrations.
- Verified…