Expose IndexSearchers executor in order to enable searcher cloning (#732)

Today if an executor was added to the IndexSearcher it's impossible to
clone the searcher with it's cache, similarty and caching policy since
the executor is not exposed. This adds a simple getter to make cloning
easier.

lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java

@@ -847,4 +847,11 @@ public class IndexSearcher {
     }
     return new CollectionStatistics(field, reader.maxDoc(), docCount, sumTotalTermFreq, sumDocFreq);
   }
+
+  /**
+   * Returns this searchers executor or <code>null</code> if no executor was provided
+   */
+  public Executor getExecutor() {
+    return executor;
+  }
 }