Implement PUB API

[dr-orlovsky]

PUB API will be used to notify connected node clients about new peer connections. It will be part of the API service from https://github.com/LNP-BP/lnpd/issues/3

PUB should have the following messages:

• CONNECTION_OPENED:connection_id:node_key:node_address
• CONNECTION_CLOSED:connection_id:node_key:node_address

NB: : sign represents not a semicolon but rather parts of ZMQ multipart message zmq::Miltipart

connection_id is the internal id of the connection used by the node and that can be used in the client API

[dr-orlovsky]

Not required for the updated architecture