GetMany implementation considerations

[i-norden]

We have a tentative GetMany interface and implementation in #1

If some of the CIDs it attempts to retrieve are not found, it currently notes the missing CIDs in a list and returns it along with a list containing the blocks it could find. In such cases where there are missing CIDs, the position of the blocks in the results slice returned will no longer be aligned with the position of the corresponding CID in the list of CIDs provided as argument. This works fine in our current usage, but we may want to consider instead including empty results into the returned results slice so that the blocks returned always have the same index as their corresponding CID.