EventStream - add Pause and State methods

[KasperZutterman]

To stop receiving new data event members midway, it should be possible to stop the EventStream. To enable this there should be the addition of the following methods:

• pause():
• importState(previousState)
• exportState()

These are needed to allow this general flow:

let stream = LDESClient.createReadStream();
stream.on("data", ...);
...
stream.pause();
let previousState = stream.exportState();

// In another run
let stream = LDESClient.createReadStream(previousState);
stream.on("data", ...);