Don't double queue async items

Author: kinyoklion

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/IterableAsyncQueue.java

@@ -14,6 +14,7 @@ public void put(T item) {
             if(nextFuture != null) {
                 nextFuture.complete(item);
                 nextFuture = null;
+                return;
             }
             queue.addLast(item);
         }