Pub-Sub API for Indy Read Replica

[rxbryan]

This is related to #27

There are two reasonable approaches to implementing this.

• WebSockets: WebSockets provide a real-time, two-way communication channel ideal for pub-sub
  APIs. Read Replicas can be notified of new transactions which they can then request from the Ledger
  or the Ledger can publish transactions to the read replicas as they become available.
• RESTful API with Queues: this approach utilizes a message queue to buffer messages and
  handle asynchronous delivery to subscribers(in our case a read replica). They are very reliable and
  would ensure that every Read Replica is are kept Update to date with new transactions whenever a
  connection can be established with them.