Update librdkafka from 1.6.1 -> 1.7.0

[GaryWilber]

I created this PR based on #879.

All the unit & e2e tests are passing for me besides the deleteTopic one, which is failing due to me testing on windows & known kafka bugs (1, 2)

Other changes:

• Added an Updating librdkafka version section to the readme to help future contributors that are interested in updating the librdkafka version. Please let me know if I missed any steps.
• Updated librdkafka-defs-generator.js script to support CRLF line endings too

[GaryWilber]

@iradul Could you review this when you get a chance? Thanks!

Also librdkafka 1.8 recently came out. I think it's still fine to move to 1.7 first. I can create another PR afterwards to move to 1.8.

[iradul]

Thanks for the PR.

[GaryWilber]

@iradul Would you be able to review this again and complete it if it looks good? I'm hoping this could get in & released soon. Afterwards I can start work on consuming v1.8.2, which contains some consumer related fixes that my service is very interested in.

[iAlex97]

Following up on this, we're in exactly the same situation as @GaryWilber 😄

I have actually forked a branch from this just so we can test if a build running with kafka 1.8.2 would solve some problems, but I didn't want to steal the credit for it.

[iradul]

Thank you @GaryWilber ! I added just a tiny fix to the CONTRIBUTING.md file.