added a shutdown method to GeoApiContext which stops RateLimitExecutorDelayThread #261

[stafichuk]

This is a pull request for issue #261. Details can be found in the comment but I will also copy them here for convenience.

The root cause of the issue is following. We are creating and starting a thread in a constructor of class RateLimitExecutorService. Here it is:

public RateLimitExecutorService() {
    setQueriesPerSecond(DEFAULT_QUERIES_PER_SECOND);
    Thread delayThread = new Thread(this);
    delayThread.setDaemon(true);
    delayThread.setName("RateLimitExecutorDelayThread");
    delayThread.start();
}

run method of this class looks like this:

@Override
public void run() {
  try {
    while (!delegate.isShutdown()) {
      this.rateLimiter.acquire();
      Runnable r = queue.take();
      delegate.execute(r);
    }
  } catch (InterruptedException ie) {
    LOG.info("Interrupted", ie);
  }
}

There are two problems with this method:

1. RateLimitExecutorDelayThread should end when the delegate is shut down but the thing is RateLimitExecutorService#shutdown method and thus shutdown() method of the delegate is never called.
2. If we call shutdown() method but the queue is empty we will wait forever in queue.take().

This issue manifests itself when you are restarting an application multiple times inside a single JVM without restarting JVM itself. For example, it can happen in application servers like Tomcat or in Play Framework's dev mode, which was my case.

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[stafichuk]

I signed it!

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