code: @Override
    public Long decrBy(final byte[] key, final long decrement) {
      checkIsInMultiOrPipeline();
      client.decrBy(key, decrement);
      return client.getIntegerReply();
    }
docs: DECRBY work just like {@link #decr(byte[]) INCR} but instead to decrement by 1 the decrement is
  integer.
  <p>
  INCR commands are limited to 64 bit signed integers.
  <p>
  Note: this is actually a string operation, that is, in Redis there are not "integer" types.
  Simply the string stored at the key is parsed as a base 10 64 bit signed integer, incremented,
  and then converted back as a string.
  <p>
  Time complexity: O(1)
  @see #incr(byte[])
  @see #decr(byte[])
  @see #incrBy(byte[], long)
  @param key
  @param decrement
  @return Integer reply, this commands will reply with the new value of key after the increment.