Range partition assigner

[pimpelsang]

Hi,

Here is range partition assigner using the same algorithm as in java client. Please review :)

closes #1564

[Saeger]

I think we have 3 levels of for loop which doesn't seem ideal, on the other hand, this is only used once assigning members to the consumers right? So it should be fine I guess. I wanted to ask if you have reasoned about this before as well?

Edit: Well, I noticed the same behaviour from the java version as well :-)

[pimpelsang]

Yes, same flow in java client. There isn't that much data to loop through and like you said it happens once. I see no reason to optimise.

[Saeger]

LGTM

[yduman]

What is preventing this PR from being merged? Our team is currently migrating from node-rdkafka to KafkaJS and since the partition.assignment.strategy is set by default to [range, roundrobin] in librdkafka, we are getting a KafkaJSProtocolError pointing to InconsistentGroupProtocolException, because KafkaJS uses by default only roundrobin. This PR would allow to fix this problem, where users can import both strategies for partitionAssigners. The alternative, without this PR, would be to copy the implementation of this solution and introduce it as a class manually by yourself.