Deferred publishing

[seoester]

This PR adds deferred publishing functionality. Both the HTTP API and the NSQ TCP API are supported.

Tests for NSQ TCP API are included. This causes (at least) 10 s of stalling during test execution.
I reduced the timeout for the publish -> consume loop from 10 s to 5 s in order to have tighter bounds for the delivery time of deferred messages. If this causes problems in the CI system, the timeout should be increased again.

[vortex14]

@seoester why we didn't answer for your PR ? What mistake you didn't resolve?
Could you resolve your conflict in your PR?

[seoester]

I think this project is in deep maintenance status, so new features are not a priority.
AFAIU, Segment has moved mostly away from NSQ for message and event delivery (sometimes to Kafka, for special cases to Centrifuge). NSQ itself also isn't developed much anymore.

If you need this PR, my recommendation is to fork segmentio/nsq-go yourself and cherry-pick all required changes on top. Then, you can use the replace directive in your go.mod to use your own fork rather than the upstream repo.