fix err in consumeOne

[sljeff]

If b.consumeOne returns an err, StartConsuming will also returns (and usually re-run due to b.GetRetry()). But:

• The two goroutines inside StartConsuming don't stop, and continue to pull messages from redis
• The data already in the deliveries will not be requeue

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To fix it, I add a channel. Before returning, it needs to:

• Close the two goroutines
• Requeue messages