Introduce `maxStartPerUserForTaskForQueue` quota

Using this key we can enforce an server wide limit that applies
per user without having to specify any users.

Example usage:
```
  [quota "*"]
    maxStartPerUserForTaskForQueue = 20 uploadpack SSH-Interactive-Worker
```

The above config would make sure that no user could perform more than
20 upload packs at any given time.

Change-Id: Iddc7d775eeee3a88ff0d8430ebb2df58a5130a93