osd: operator<< set<shard_id_t> is a list of numbers

Specialize operator<< set<shard_id_t> to not output char elements (which
is the default because it is an uint_8) but a list of numbers.

http://tracker.ceph.com/issues/8254 Fixes: #8254

Signed-off-by: Loic Dachary <loic@dachary.org>