Consuming messages from each partition separately

[martijnimhoff]

Closes #1063

Notes:

• Currently we're running our fork in production which is using the consume per partition method.
• I've updated the documentation and the examples to simplify getting started with this way of consuming messages.
• While debugging the code, I discovered that some of the callbacks return values which are not specified in the Typescript types. I've made changes where needed.

There are no breaking changes, therefore this could be released as a minor version upgrade. We would appreciate if this was merged so that we can remove our fork and use this package directly in production.