mon/osdmonitor: decouple adjust_heartbeat_grace and min_down_reporters
if cancel mon_osd_adjust_heartbeat_grace, we must set mon_osd_min_down_reporters to zero.
otherwise the next checking for
reporters_by_subtree.size() >= g_conf->mon_osd_min_down_reporters
will allways be fail.the two options should take effect respectively...
The pull request has been merged to master branch, see: https://github.com/ceph/ceph/pull/8749