Add ability to pause consumer

[Nevon]

Consumer should expose pause and resume methods, accepting a list of topic-partitions. When a topic-partition is "paused", no data will be fetched from that topic-partition.

One use case is to be able to add a kill switch to a consumer to quickly be able to stop consuming from that partition.

Q: How should this affect partition reassignment?

[Nevon]

All the other clients I have looked at seem to expect a list of topic-partitions, but it doesn't seem like a very common use-case that you would need to pause only a subset of partitions.

I think for a first implementation, I will just accept a list of objects with topic as the only field:

consumer.pause([{ topic: 'topic-name' }])

That way we can be backwards compatible if we in the future want to allow:

consumer.pause([{ topic: 'topic-name', partitions: [1,2,3] }])