rgw: data sync checks empty next_marker for datalog

rgw: data sync checks empty next_marker for datalog

RGWReadRemoteDataLogShardCR tracks the marker and next_marker
separately, because next_marker will be empty when it reaches the end

this allows RGWDataSyncShardCR to avoid clearing its sync_marker and
restarting datalog listing from the beginning

Fixes: http://tracker.ceph.com/issues/39033
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 2ff63778342282ab5c676529f6e7d90f30e6da1a)

Conflicts:
	src/rgw/rgw_data_sync.cc: no sync tracing