A delayed fetch request, which is satisfied (or more
accurately, unblocked) -- if:
Case A: This broker is no longer the leader for some partitions it tries to fetch
should return whatever data is available for the rest partitions.
Case B: This broker is does not know of some partitions it tries to fetch
should return whatever data is available for the rest partitions.
Case C: The fetch offset locates not on the last segment of the log
should return all the data on that segment.
Case D: The accumulated bytes from all the fetching partitions exceeds the minimum bytes
A delayed fetch request, which is satisfied (or more accurately, unblocked) -- if: Case A: This broker is no longer the leader for some partitions it tries to fetch