…e random_

To remove duplicates, I had to:
 - introduce a new struct `UidKey` to store the index of a UID in the
   matrix
 - calculate total number of nodes and check if required number of nodes
   is less than that. If it's not, return all the nodes.
 - one caveat is that the order is not randomized when the requested
   number is equal to total number of nodes.

the random uids are now selected from each uid list instead of
the uid matrix. This simplifies the process.