@Override
  public void closeProcessor() {
    try {
      super.closeProcessor();
    } finally {
      releaseUnprocessedEvents();
    }
  }
Handle failover. This method is called when a secondary <code>GatewaySender</code> becomes a
primary <code>GatewaySender</code>.

Once this secondary becomes the primary, it must:
<ul>
<li>Remove the queue's CacheListener
<li>Process the map of unprocessed events (those it has seen but the previous primary had not
yet processed before it crashed). These will include both queued and unqueued events. Remove
from the queue any events that were already sent
<li>Clear the unprocessed events map
</ul>
geode-core/src/main/java/org/apache/geode/internal/cache/wan/serial/SerialGatewaySenderEventProcessor.java