authorizer: fix bugs and expire cache

Since the server wipes out existing users when their authorization
expires, periodically rerun checks for cached users and revoke their
authorization if the checks fail.

Also, make sure we only have one checker live for users at a time.
Resolves the issue where a user would be added to firewall rules
multiple times.

Change-Id: I05685e93198a0bc21fe6e2ac52e0cb6db90f2691
6 files changed