FCPClientReferenceImplementation: Use EventSource instead of Persistent

The EventSource interface is an abstraction of class Persistent.
EventSource was introduced specifically only for event-notifications, which the
FCPClientReferenceImplementation solely is about right now.
DebugFCPClient: Use EventSource instead of Persistent

The EventSource interface is an abstraction of class Persistent.
EventSource was introduced specifically only for event-notifications,
which the DebugFCPClient solely is about right now.
SubscriptionManagerFCPTest: Use EventSource instead of Persistent

The EventSource interface is an abstraction of class Persistent.
EventSource was introduced specifically only for event-notifications,
which the SubscriptionManagerFCPTest solely is about right now.