[Tilesathome] Maximum active requests per user implemented

[spaetz]

Out of 69 active users, they have so many active requests simultanously:
|  20 | 
|  23 | 
|  25 | 
|  29 | 
|  32 | 
|  34 | 
|  50 | 
|  55 | 
|  75 | 
|  94 | 
|  96 | 
| 197 | 
| 304 | 
| 660 | 

It seems that some clients just continue taking jobs preventing everyone else from rendering them for the next 6 hours. To prevent this, I have now implemented a maximum of 50 active requests per user. If you have a render farm larger than this, please use a second user id :-).

The downside is, that in case of an error, a request can well continue to count as active until it times out 6 hours later. We need to address this with a better error feedback, so that your active count gets reduced when that happens. But hopefully this means better rendering for everyone without me having to observe the active list manually all the time.

As always, feedback and patches welcome,
spaetz