API for event stream and subscription management

[jimthematrix]

Each eventstream represents a configured connection from the fabconnect server to a client listener. The connection can be one of the following types:

• webhooks: the client must implement a POST endpoint that may be protected by an authorization header (basic or bearer tokens), for the events to be delivered as JSON payload
• websocket: the client initiates a websocket connection for the events to be streamed

Under a particular event stream, multiple subscriptions can be defined, each targeting a named chaincode event, supported with regular expression based matching.

This will use the leveldb defined under the EventStreamConf object to store the events and subscriptions

[jimthematrix]

fixed by #41