msg: add min delay packets support for mons

msg: add min delay packets support for mons

when suffering network bottleneck, like switch qos control for only 50Mbps,
mon cluster may stuck in electing for long time and elects again and again,
which introduces many troubles, like it's hard to find where the problem is.
even more worse, it may lead to IO hang when one osd can't serve IO.
so it's better to keep mon cluster stable always.

Signed-off-by: Song Shun <song.shun3@zte.com.cn>