DebugFCPClient: Pass database to new event handlers

They cannot obtain it themselves: The type parameter T is erased at
runtime due to the way Java is implemented.
DebugFCPClient: Logging

Inspired by SubscriptionSynchronizationHandlerImpl which is being
replaced by these new handlers.
DebugFCPClient: Store if a synchronization is in progress

Will be used by the SubscribedObjectChangedHandlerImpl for determining
how to validate the received objects.