LavinMQ::AMQP::DelayedQueue::DelayedMessageStore

A delayed queue must have its messages ordered by "expire at" so the message expire loop will look at the right message. To achieve this, messages are always added to a custom requeued store, which acts as an in-memory index ordered by when messages should be published. The requeued store is used because #shift and #first? look for requeued messages first, then read the next from disk. For a delayed queue we never want to read messages in the order they arrived (were written to disk).