Sample partition's uids

[EnricoMi]

Check with Dgraph devs if they could add an operation to GraphQL that provides you a sample of the uids that match a query. Retrieving only every N-th uid could be fed into the uid range partitioner to get proper partitions. Right now we split the possible space of uids into partitions. With the sample we could split the actual space of uids per partition.

An approximation of the sample would also help and might scale sub-linearly. See Sample your result.