anchor.diff:  
   Sometimes an exception might occur in your notification plugin due to which it might not be able to handle the event sent to it by Kill Bill. By default, if a plugin triggers a runtime exception, Kill Bill dispatches the event right away up to 3 times (or as configured by the `org.killbill.notificationq.external.max.failure.retry` global property). However, in some cases, you may want Kill Bill to retry sending the event again at a later time (if for example a third-party provider is down). To do so, your plugin can throw a `NotificationPluginApiRetryException` to include its own retry schedule. The retry schedule should include a Period array, each element in the array should specify the duration after which the retry should be attempted. 
   
  -For example consider the following exception:
  +For example, consider the following exception:
   [source,java]
   // Retry in an hour and in 24hrs
   throw new NotificationPluginApiRetryException(Arrays.asList(new Period[]{Period.hours(1), Period.days(1)}));
  
positive.code: |ACCOUNT_CHANGE
  |Occurs when a customer account is modified
  |BLOCKING_STATE
  |Occurs when there is a change in the state of an entitlement or billing . See https://killbill.github.io/slate/#account-blocking-state[Blocking States] to know more
  |BROADCAST_SERVICE
  |Used to broadcast an event to other Kill Bill nodes (Typically used for plugin related events like plugin installation, plugin uninstallation, plugin start, plugin stop, etc. )
  |SUBSCRIPTION_CREATION
  |Occurs when a new subscription is created
  |SUBSCRIPTION_PHASE
  
negative.code: 
  When the `NotificationPluginApiRetryException` is caught by Kill Bill, the system computes the next retry date based on the schedule specified in the exception and the number of times that specific event has been retried. 
  
  So, in terms of responsibilities:
  
  * Plugin is in charge of deciding whether a `NotificationPluginApiRetryException` should be thrown and attach the associated retry schedule to it.
  * Kill Bill manages the retry logic and also keeps count of # existing retries versus retry schedule.
  