Redis Mirror available capacity key is dropping below 0

pablo [11:31] 
```irb(main):001:0> Gitlab::Mirror.available_capacity
=> -361
irb(main):002:0> Gitlab::Mirror.available_capacity
=> -442
irb(main):003:0> Gitlab::Mirror.available_capacity
=> -581

What this prompt tells us is that stuff is being scheduled without actually existing capacity for it, which should not be happening.

\cc @DouweM @pcarranza