Remove session limits and sequencing

Summary:
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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310, T3720

Differential Revision: https://secure.phabricator.com/D7978