rgw: fix transition from full to incremental meta sync

rgw: fix transition from full to incremental meta sync

when transitioning to incremental sync, we need to write temp_marker
instead of sync_marker, because temp_marker has replaced the full sync
marker (which is a metadata key, rather than an log marker) with
next_step_marker

this was breaking the metadata checkpoint in test_multi.py, because it
was comparing metadata keys (i.e. 'user:zone.user') against mdlog
markers

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