Make murmur2 implementation compatible with Java version

[Nevon]

Fixes #357

There were two main issues. The first one being that Java does integer division, so length4 would be 2.25 in JS vs 2 in Java, which meant that we would do an additional iteration in the loop compared to the Java version.

The second, and far more annoying issue, is that Java's int and Javascripts Number type behave quite differently, so we'd get wildly different results during multiplication of large numbers. I changed the implementation to use Long, which solved the problem. I'm not very happy about the part where I convert back to an int to do a zero fill right shift, but I found that Long's k.shiftRightUnsigned(r) behaves differently to k >>> r (Java), but maybe I misunderstood something.

I haven't updated the existing murmur2 tests, since they will now all fail. This feels like a super breaking change, so I'm not sure how we should roll it out. I'm opening this PR for the sake of discussion.

@pimpelsang

[Nevon]

My concern with changing this in the default partitioner is that new messages produced with this version will end up in different partitions, which may break some applications that (quite reasonably) expect all messages for a given key to be assigned to the same partition.

One option could be to expose this as a separate partitioner that @pimpelsang and anyone else requiring compatibility with the Java partitioner can use, and then in a later major version change switch to that being the default.

EDIT: Github thinks I'm from the future. This comment was actually posted right after creating the PR, long before there were any comments. 🤔

[tulios]

I agree @Nevon, we can't change the current murmur implementation without breaking compatibility. I think we should create a murmur2Java implementation and a defaultJava partitioner that uses the new algorithm and then document that default doesn't follow the same logic as the Java partitioner, but it's easy to switch to defaultJava. When I read the issue I was sure that it was a number overflow, Long got into the project quite late. I think the murmur implementation was already in place and never got reviewed.

[Nevon]

Sounds good. I'll go for that solution. Will update the PR.

[Nevon]

Not sure why Github thinks I live in the future (the comment after this was actually posted before Tulio's...)

I've exposed both the default partitioner and the Java compatible one now, and documented how to use them and why you might want to:

const { Partitioners } = require('kafkajs')
kafka.producer({ createPartitioner: Partitioners.JavaCompatiblePartitioner })