kvstreamer: refactor the loop of processing the batch response

This commit refactors the code which processes the batch response in
order to separate out two different concerns:
- processing non-empty responses in order to create `Result`s
- processing incomplete responses to populate the resume request.

Now each of these "concerns" is handled in a separate loop making it
easier to reason about, especially so in the following commit.

This commit also extracts out multiple return arguments of a function
into a struct as well as updates some of the comments and moves some of
the error-checking to an earlier stage.

Release note: None