Consuming messages from each partition separately

A Consumer that is subscribed to multiple partitions cannot control the mix of messages it consumes from each partition.
There are various reasons why controlling that is relevant, see eg. #719 and https://github.com/confluentinc/librdkafka/wiki/FAQ#what-are-partition-queues-and-why-are-some-partitions-slower-than-others.

That last link shows that librdkafka supports consuming from each partition separately. It would be great if that implementation is available in node-rdkafka as well. An example could be a `PartitionConsumer` class, that does not have the normal `.consume` methods, but `.consume(topic, partition, number, callback)`.

Start with the existing Consumer entry points and the librdkafka partition-queues FAQ, then review the context in #719. Define how a PartitionConsumer would expose `.consume(topic, partition, number, callback)` and verify that consumers subscribed to multiple partitions can control each partition independently.

Written by the indexing model from the issue text.