Log when messages cache detects topology change (#354)

Co-authored-by: Moxie Marlinspike <moxie+github@signal.org>

service/src/main/java/org/whispersystems/textsecuregcm/storage/MessagesCache.java

@@ -127,6 +127,8 @@ public class MessagesCache extends RedisClusterPubSubAdapter<String, String> imp
     }
 
     private void resubscribeAll() {
+        logger.info("Got topology change event, resubscribing all keyspace notifications");
+
         final Set<String> queueNames;
 
         synchronized (messageListenersByQueueName) {