[Access] Implement SubscriptionHandler interface and CreateSubscriptionHandler factory

[Guitarheroua]

SubscriptionHandler Implementation

Based on the Draft design of new WebSockets, a new SubscriptionHandler interface should be implemented as part of the new WebSocket subscription system, building on the current WebSocketController. All requirements and relationships between components are described in the draft design document.

The SubscriptionHandler interface abstracts the actual subscriptions managed by the WebSocketBroker. Concrete SubscriptionHandler implementations will be created during the WebSocketBroker::subscribe call, depending on the topic provided by the client. For example, the topic events will have an EventsSubscriptionHandler implementation to manage event subscriptions.

The supported topics include:

• events
• account_statuses
• blocks
• block_headers
• block_digests
• transaction_statuses

All concrete handlers will be implemented in separate tasks, based on this interface.

This issue includes implementing the interface and a factory method:

type SubscriptionHandler interface {
    Close() error
}

func CreateSubscriptionHandler(topic string, arguments map[string]interface{}, broadcastMessage func([]byte) error) (SubscriptionHandler, error)

If possible, a basic implementation of the SubscriptionHandler interface should be provided, which will include common functionality for each concrete handler. This can be done after some of the handlers have already been implemented.