feat: consume per partition

[martijnimhoff]

Closes https://github.com/confluentinc/confluent-kafka-javascript/issues/147

[confluent-cla-assistant[bot]]

:tada: All Contributor License Agreements have been signed. Ready to merge.
:white_check_mark: martijnimhoff
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}