code: @Override
    public Long zrank(final byte[] key, final byte[] member) {
      checkIsInMultiOrPipeline();
      client.zrank(key, member);
      return client.getIntegerReply();
    }
docs: Return the rank (or index) or member in the sorted set at key, with scores being ordered from
  low to high.
  <p>
  When the given member does not exist in the sorted set, the special value 'nil' is returned.
  The returned rank (or index) of the member is 0-based for both commands.
  <p>
  <b>Time complexity:</b>
  <p>
  O(log(N))
  @see #zrevrank(byte[], byte[])
  @param key
  @param member
  @return Integer reply or a nil bulk reply, specifically: the rank of the element as an integer
  reply if the element exists. A nil bulk reply if there is no such element.