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. There's no functional change -- the code to generate sessions guarantees that it will never write duplicate rows or write additional rows -- but allows us to drop the `-1`, `-2` qualifiers in the future.
  - Also of note, our task is made far simpler here because MySQL will automatically assign values to new `AUTO_INCREMENT` columns, so we don't have to migrate to get real IDs.

Test Plan: Ran migrations, verified table looked sane. Logged out, logged in.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3720, T4310

Differential Revision: https://secure.phabricator.com/D7975