Add max_retries option to the configuration

This option makes possible to set a maximum number the plugin will retry writes
for. It defaults to retry forever (negative max_retries) to keep backward
compatibility.

Author: arichiardi

lib/logstash/outputs/mongodb.rb

@@ -202,7 +202,7 @@ def receive(event)
         end
       else
         result = write_to_mongodb(collection, [document])
-        @logger.debug("Bulk write result", :result => result)
+        @logger.debug("Bulk write result: #{result.to_s}")
       end
 
     rescue => e