code: @Override
    public Long sunionstore(final byte[] dstkey, final byte[]... keys) {
      checkIsInMultiOrPipeline();
      client.sunionstore(dstkey, keys);
      return client.getIntegerReply();
    }
docs: This command works exactly like {@link #sunion(byte[]...) SUNION} but instead of being returned
  the resulting set is stored as dstkey. Any existing value in dstkey will be over-written.
  <p>
  Time complexity O(N) where N is the total number of elements in all the provided sets
  @param dstkey
  @param keys
  @return Status code reply