Application Client Backpressure: Pause/Resume via Kafka Consumer API

[dmytro-landiak]

Pull Request description

#290

Previously, application client consumers were fully stopped and restarted on channel backpressure events. This had a bug where shared subscription consumers were not being resumed after the channel became writable again, causing message lag to grow continuously until the client reconnected.

This PR replaces the stop/start approach with Kafka's built-in pause()/resume() consumer API. Both main and shared subscription consumers are now paused when the channel becomes non-writable and seamlessly resumed when it recovers — without any consumer group lifecycle overhead. This leads to more efficient system resource usage, eliminates the lag accumulation bug in shared subscription processing, and improves overall throughput stability under backpressure conditions.

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