Summary:
This prevents security by obscurity.
If I have read-only access to the database then I can pretend to be any logged-in user.
I've used `PhabricatorHash::digest()` (even though we don't need salt as the hashed string is random) to be compatible with user log.
Test Plan:
Applied patch.
Verified I'm still logged in.
Logged out.
Logged in.
$ arc tasks
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D6080
