Integrate statesPub interface into client.

[manoranjith]

Location

[client]

Context

Refactor of watcher for IoT use cases is described in umbrella issue: #172. To integrate the refactored watcher into the client, each channel should publish the off-chain transactions to the watcher via StatesPub interface.

type StatesPub interface {
    Publish(channel.Transaction) error
}

Details

New states are created for the Channel when enabling newer off-chain states. In this place, the newer state should be published to the watcher.

However, the statesPub instance will be available only after a channel is initialized and the client registers the channel with the watcher. Therefore,

1. Add a field Publisher with type func(channel.Transaction) error to the client.Channel type.
2. Implement a NoopPublisher that always returns nil.
3. When initializing the channel, set the Publisher to a NoopPublisher.
4. Publish the states on the Publisher when enabling newer off-chain states.

In an upcoming pull request, NoopPublisher will be replaced by the Publisher associated with the watcher when client registers the channel with the watcher.