rgw: meta log rest handlers avoid get_log()

rgw: meta log rest handlers avoid get_log()

RGWMetadataManager::get_log() will allocate a log and keep it in memory.
this could lead to a potential denial of service by making requests with
lots of different period ids

RGWMetadataLog if effectively stateless (the only state is a set of
modified_shards, which are not touched by any of the rest api calls), so
we can use a temporary instead of calling get_log()

Signed-off-by: Casey Bodley <cbodley@redhat.com>