Refactor clients based on ProtocolEngine

The ProtocolEngine interface allows certain tasks to be performed in response to local actions and incoming messages: updating local state, sending messages, broadcasting events and deleting the incoming message. Clients based on ProtocolEngine that need to perform other tasks are forced to store a task label in the local state, then retrieve it and perform the task outside the engine. This defeats the purpose of the engine interface, which is meant to encapsulate the protocol logic.

Alter or remove the ProtocolEngine interface so clients have the flexibility they need.