Increase default number of subscribers

Multi-site plugin is using additional topic for batch indexing events.
To prevent threads starvation default number of subscribers is
increased.

Feature: Issue 12228
Change-Id: Iee3262d67dfc8b050c16bf4088dd9a730b6c021d
diff --git a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
index c012081..c93db20 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
@@ -24,7 +24,7 @@
 public class KafkaSubscriberProperties extends KafkaProperties {
   private static final long serialVersionUID = 1L;
   private static final String DEFAULT_POLLING_INTERVAL_MS = "1000";
-  private static final String DEFAULT_NUMBER_OF_SUBSCRIBERS = "4";
+  private static final String DEFAULT_NUMBER_OF_SUBSCRIBERS = "6";
 
   private final Integer pollingInterval;
   private final String groupId;