Graceful termination when there are more I/O nodes

[SalvatoreDiGirolamo]

Each I/O node sends an HELLO message to the DV and all of them have the same ID (i.e., jobid).
After the termination of the first I/O node, all the others become non authorized.
Solve by adding a ref. count to the simulation and terminate it only when the same number of termination messages are received.