osd: restart peering if requesting acting osd goes down

osd: restart peering if requesting acting osd goes down

If we request an acting set, we need to restart peering if one of the
requested nodes goes down.  This prevents a deadlock where we get stuck
in WaitActingChange because we have [a,b], want [a,b,c], but c is down and
our up and acting don't actually change.

Signed-off-by: Sage Weil <sage@newdream.net>