mon: OSDMonitor: only share osdmap with up OSDs

Try to share the map with a randomly picked OSD; if the picked monitor is
not 'up', then try to find the nearest 'up' OSD in the map by doing a
backward and a forward linear search on the map -- this would be O(n) in
the worst case scenario, as we only do a single iteration starting on the
picked position, incrementing and decrementing two different iterators
until we find an appropriate OSD or we exhaust the map.

Fixes: #3629
Backport: bobtail

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>