Requeuer and Message Delay

[m110]

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• Added the Requeuer component.
  • It works as a simpler version of the Forwarder, routing messages from one topic to another (a dynamic one).
  • Can be used to move messages that failed to process back to the original topic, so they don't block other messages.
  • Can be used together with the poison middleware and the delay component to delay the forwarding.
• Added the delay package. It contains helpers for setting delay metadata on messages.
  • Does not do anything by itself. A Pub/Sub needs to support it explicitly. For now, that's the delayed postgres Pub/Sub implemented in https://github.com/ThreeDotsLabs/watermill-sql/pull/34
  • Use case 1: publishing a message after a given delay or at given time (see the example).
  • Use case 2: automatically moving messages out of the poison queue to the original topic after a delay (used together with the Requeuer component).
• Added the pq CLI tool for working with poison queues.