fix(batch): start the context before returning it

Author: bevzzz

src/main/java/io/weaviate/client6/v1/api/collections/batch/WeaviateBatchClient.java

@@ -47,6 +47,7 @@ public BatchContext<PropertiesT> start() {
       context.scheduleReconnect(GCP_RECONNECT_INTERVAL_SECONDS);
     }
 
+    context.start();
     return context;
   }