Optimize consumer poll loop to skip sleep when active

[anilcan-kara]

This PR addresses issue #173 by optimizing the consumer loop to avoid unnecessary sleeps when deliveries are successfully prefetched.

Key Changes:

• consumeBatch(): Now returns (int, error), representing the count of successfully prefetched deliveries in the batch and any Redis error.
• consume(): Updated the consumption loop to only execute the time.Sleep wait block when count == 0 (meaning either the queue is empty, or the local prefetch delivery channel is full).

Benefits:

• Throughput & Latency: When there are active messages in the queue and consumers have capacity, they are prefetched and delivered instantly without the pollDuration sleep delay.
• Redis Load: When the queue is empty, the consumer loop still sleeps for pollDuration, keeping the polling frequency low and conserving Redis CPU resources.

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