Now that acknowledge returns a Promise, should negativeAcknowledge return a Promise too?

The `acknowledge` method of the `Consumer` type was [recently updated](https://github.com/apache/pulsar-client-node/pull/200) to return a `Promise`. Should the `negativeAcknowledge` method also return a `Promise`?

Start with the Consumer type's acknowledge and negativeAcknowledge methods, then read pull request #200 to understand the earlier Promise change. Determine whether negativeAcknowledge should match that API and identify the relevant implementation and tests; done means the decision is settled and any agreed API change is covered.

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