* Move sessions to user_login metadata, as that data rarely changes after login.
* Merge user and channel access hashes. Those IDs don't conflict.
* Split usernames into a new table to allow better `ON CONFLICT` updates
(when a username moves to another entity, we want the old row to be replaced).
Usernames also don't need to be scoped to a login.
Tulir Asokan
Sumner Evans