R1:a64228b03fbc

Give the session table a normal `id` column as a primary key

Summary:
Ref T4310. Ref T3720. Two major things are going on here:

- I'm making this table work more like a standard table, which, e.g., makes `delete()` simpler to implement.
- Currently, the primary key is `(userPHID, type)`. I want to get rid of this, issue unlimited sessions, and GC old sessions. This means we can't have a unique key on `(userPHID, type)` anymore. This removes it as the primary key and adds it as a normal key instead.