Support Electrum protocol v1.7 #231

The new v1.7 protocol version is still a working in progress, see: spesmilo/electrum-protocol#2

We are particularly interested in the proposed subscription option as this would be helpful to clients tracking when a tx is commited:
blockchain.outpoint.subscribe() to subscribe to a transaction outpoint, and get a notification when it gets spent.

It should be further discussed and addressed after #172