osd/PG: async-recovery should respect historical missing objects

osd/PG: async-recovery should respect historical missing objects

Peers with async-recovery enabled are usually having a update-to-date
last-update iterator and hence might be moved out from the __async_recovery_targets__
set during the next peering circles.

7de35629f562436d2bdb85788bdf97b10db3f556 makes num_objects_missing
trace historical missing objects correctly, hence we could take
num_objects_missing into account when determing __async_recovery_targets__.

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