Do connect if connection is none on getChannel()

This can reconnect session even if connection suddenly disconnected.
diff --git a/src/main/java/com/googlesource/gerrit/plugins/rabbitmq/AMQPSession.java b/src/main/java/com/googlesource/gerrit/plugins/rabbitmq/AMQPSession.java
index c0215f5..ce0f729 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/rabbitmq/AMQPSession.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/rabbitmq/AMQPSession.java
@@ -55,6 +55,9 @@
 
   private Channel getChannel() {
     Channel ch = null;
+    if (connection == null) {
+      connect();
+    }
     if (connection != null) {
       try {
         ch = connection.createChannel();