Perform update checks concurrently

[eloquence]

On my system (3rd gen X1 Carbon), the "Checking for workstation updates" stage currently takes about 4 minutes, even when no updates are available (I stopwatched it). That's a long time to ask a journalist to wait before being able to use the client. It would be good to find ways to get this number down. A concurrency of 2 or greater has been suggested to improve update performance.

[zenmonkeykstop]

Deferring to upstream changes in the Qubes updater.