Groups the
requests into a
RequestGrouping, which contains:
keyed requests: requests that include a key and it is therefore possible
to determine to which master node they should be sent; all requests in each inner list
in the
keyed collection are guaranteed to be sent to the same
master node;
unkeyed requests: requests that do not include a key and it is therefore
not possible to determine to which master node they should be sent.
If any of the
requests includes multiple keys that belong to different master nodes,
the resulting future will fail.