Services are forced to implement callback handlers

[ronnno]

Spec enforces all services to implement handler interfaces.
Handler may be implemented by a dependency of the service rather than the service itself. this allows separation of concerns. current implementation of code generation in Membuff - it imposes a design decision - it confounds inter node and intra node communication (forces them to have the same mechanism)

for instance, PublicApi implements TransactionResultsHandler and currently just delegates the call to a synchronization subsystem.