Do not make the Poller/Receiver daemon mandatory for a realm

[mohierf]

As of now, every realm must have a scheduler, a broker and a poller daemon. The broker is able to manage_sub_realm and thus only one instance of this daemon is needed but the poller must be launched for every declared realm.

If no active checks are launched , the poller (and its worker processes) is never used !

When the configuration is parsed and splitted across the realm, the arbiter may check if some active checks are to be used and then ignore the poller if it is not needed in a realm

[mohierf]

For the same reason, having a receiver in a realm when no passive checks are expected is also of no interest

[mohierf]

Note that even the broker may not be mandatory if no modules are to be attached