osd/OSD.cc: choose heartbeat peers by failure domain

osd/OSD.cc: choose heartbeat peers by failure domain

By default, monitor requires at least two valid failure votes/reports from
different hosts to mark an OSD down, which turns out to be impossible sometimes
for a replicated-pool of size of 2 in those clusters made up of hosts
with contiguous labeled OSDs.

This patch instead does a breadth-first search based on the highest level of
failure domain at cluster-wide, to try to make heartbeat peers can cover all failure domains
whenever possible, which can hopefully help accelerating osd failure detection
in the above case..

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