Add a publish/subscribe api for blockchain events

[ecioppettini]

Add a publish/subscribe api for blockchain events

Idea

Have a node service/module that allows to subscribe to blockchain events
that can be used for things like an external explorer/blockchain indexer, wallet backend, etc.

Interesting Events

• New block
• Tip changed

Steps

1. Create a new message box in the node just like in the explorer, and
   propagate the necessary events (one of those is basically what's done for
   the explorer right now).

2. Add a new warp handler for websockets at ws://host/notifier

   Basic idea

   • the handler will listen for new connections
   • at each local event will send a message to all the
     connected clients

   The messages will be (for now)

   • NewBlock(blockid): Passing id's here means that the actual block will
     need to be obtained from somewhere else, but that's already needed
     for bootstrapping (getting blocks from 0..tip), so it may be
     enough to solve that problem there
   • NewTip(blockid)

   All messages are one-directional. Node -> Connected clients. Clients can't
   publish things.

3. Add new settings to configuration.

max_connections: maximum number of subscribers at the same time

Questions

Do we want the clients to be able to subscribe to specific
messages/topics/channels?
How should we encode the messages? Using a binary format? text? If we only send
ids it's not much of a difference, though.

[gufmar]

I have no experience with ws and so not sure if this makes sense, but reading about 'able to subscribe to specific messages/topics/channels' let me think about MQTT. Their clients and brokers already implement topics and subscriptions. There are plenty of client implementations. Lightweight. Standardized.

[gufmar]

Looks like websockets and mqtt can be combined: http://www.steves-internet-guide.com/mqtt-websockets/