fix #2850 Enable batch mode in shard allocator to suppress WARN log for single shard assignment fallback

src/main/resources/esclient.xml

@@ -7,6 +7,7 @@
 			{"http.cors.enabled":"true",
 			 "http.cors.allow-origin":"*",
 			 "discovery.type":"single-node",
+			 "cluster.allocator.existing_shards_allocator.batch_enabled":"true",
 			 <!--
 			 "node.name":"search_engine",
 			 "discovery.seed_hosts":"search_engine",