Refactor Pusher Engine

[tim-barry]

Problem Definition

The Pusher Engine should be updated to clarify its interface, including blocking versus non-blocking calls, error handling and propagation, documentation, and that it does not process messages from the network (only from the node). See comments on #6747 for more details.

Proposed Solution

• Replace use of the deprecated engine.Unit with ComponentManager
• Converting the pusher engine from implementing the network.Engine interface to instead using the network.MessageProcessor interface, but not actually accepting any messages from the network because the pusher.Engine is fed only from internal components
  • use a type-specific function/interface for interaction from the Finalizer
• Ensure unit tests accurately reflect the necessary functionality of the pusher engine