Limit the number of concurrent tasks by ponos agent

A ponos agent should never run more tasks than its number of reported cores.

Currently the agents in prod are running 300+ processes each of IIIF import. This kind of task is really low on CPU usage individually, but is time consuming (network bound). It's useless to start more tasks than the nb of cores, as the system cannot really process tasks much faster, and it overloads a bit the prod db