common/config: change type of osd_mon_ack_timeout from int to double

common/config: change type of osd_mon_ack_timeout from int to double

We may use osd_mon_ack_timeout the following way:
  double backoff = stats_ack_timeout / g_conf->osd_mon_ack_timeout;
which is at risk of precision lost.

Example:

main()
{
  double r = 5 / 30;
  cout << "r = " << r << std::endl;
}

Output:
  r = 0

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>