Remove session limits and sequencing
Ref T4310. Fixes T3720. This change:
- Removes concurrent session limits. Instead, unused sessions are GC'd after a while.
- Collapses all existing "web-1", "web-2", etc., sessions into "web" sessions.
- Dramatically simplifies the code for establishing a session (like omg).
Test Plan: Ran migration, checked Sessions panel and database for sanity. Used existing session. Logged out, logged in. Ran Conduit commands.
Reviewed By: btrahan
Maniphest Tasks: T4310, T3720
Differential Revision: https://secure.phabricator.com/D7978