AT

• when the client gets bootstrapped with the ExternalTaskClientBuilder, I can pass an object which implements a custom back off strategy
  • this option is not mandatory
• the back off strategy must implement an interface provided by the external task client
  • the interface specifies a method signature which is invoked in TopicSubscriptionManager#acquire if at least one subscription is present after invoking handlers
• there is no default implementation provided by the external task client