SSEStream status monitoring

[lijamie98]

What problem does your feature solve?

While implementing the health check of the Stellar Platform Server, we need to check if the stream's executor service is live and running properly.

What would you like to see?

Make the status of the ExecutorService available.

Create getters for the following statuses:

  private final AtomicBoolean isStopped = new AtomicBoolean(false);
  private final AtomicBoolean serverSideClosed = new AtomicBoolean(true); // make sure we start correctly
  private final AtomicReference<String> lastEventId = new AtomicReference<String>(null);

https://github.com/stellar/java-stellar-sdk/blob/master/src/main/java/org/stellar/sdk/requests/SSEStream.java#L33-L35

What alternatives are there?