osd/PG: find_best_info - add completeness as the preferred option

osd/PG: find_best_info - add completeness as the preferred option

Async recovery peers usually have a relative complete
log history but may exist a lot of missing objects. Choosing them
as auth_log_shard and further as primary if current up_primary is
unrecoverable, say, could have a bigger chance to block client I/Os.

Among peers with identical new log history, we now consider those
who are now complete (having no missing objects) as the preferred
ones when determining auth_log_shard.

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