Bump franz-go deps and refactor code to handle breaking changes

[ekeric13]

Take 2 of: https://github.com/benthosdev/benthos/pull/1361

Main purpose for upgrading is to handle a bug I was having in an older version of franz-go that I have at least yet to reproduce on the newer version. The maintainer, Travis, was very responsive and even upgraded franz-go to handle an edge case that we were hitting with committing empty offsets: https://github.com/twmb/franz-go/issues/186

Main breaking change was the fact that PollFetches now returns a deadlineExceeded error if the context is cancelled, which it is after 1 second.

I do want to add some test this go around but please let me know @Jeffail if this is the right direction.