R1:acb141cf52c1

Expire and garbage collect unused sessions

Summary:
Ref T3720. Ref T4310. Currently, we limit the maximum number of concurrent sessions of each type. This is primarily because sessions predate garbage collection and we had no way to prevent the session table from growing fairly quickly and without bound unless we did this.

Now that we have GC (and it's modular!) we can just expire unused sessions after a while and throw them away:

- Add a `sessionExpires` column to the table, with a key.
- Add a GC…