rgw/multisite: in order to sleep between mdlog polling events, we check if the mdlog_...

rgw/multisite: in order to sleep between mdlog polling events, we check if the mdlog_marker is not modified by comparing
mdlog_marker and max_marker. but max_marker is exposed to changes from RGWReadMDLogEntriesCR, and if there is a race
coming from mdlog trimming which could render max_marker empty, then its comparison with mdlog polling can be incorrect.
To fix this, we now save the previous mdlog marker and compare with the updated mdlog marker.

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>