Define a maximum number of cuncurrent tasks in NPrinting
NPrinting does not have a limit to running tasks but, I suppose, it's limit are calculated by number of CPUs; if the server has 8 CPUs it could elaborate 8 tasks at same time but 8 time slower than a single one.
In case of on-demand distributed to the users this often occurs.
I need a parameter to set a maximum number of cuncurrent tasks, the others go to queue.
