code: @Override
    public Long append(final byte[] key, final byte[] value) {
      checkIsInMultiOrPipeline();
      client.append(key, value);
      return client.getIntegerReply();
    }
docs: If the key already exists and is a string, this command appends the provided value at the end
  of the string. If the key does not exist it is created and set as an empty string, so APPEND
  will be very similar to SET in this special case.
  <p>
  Time complexity: O(1). The amortized time complexity is O(1) assuming the appended value is
  small and the already present value is of any size, since the dynamic string library used by
  Redis will double the free space available on every reallocation.
  @param key
  @param value
  @return Integer reply, specifically the total length of the string after the append operation.