A Durable Subscription allows the consumption of messages that are
sent while the subscriber, which can be a message-driven bean, is not active.
A subscription is registered with a unique identity, which is retained by JMS.
Subsequent subscriber objects with the same identity resume the
subscription in the state that it was left by the previous subscriber with
the same identity.
Durable subscription does have some overhead, but the client does not have
to ensure that the message-driven bean subscription is active.