rgw, cls_rgw: clean up olh artifacts when dropping last version

This is still missing one piece, however, it works like this:
1. bucket index log reflects last version was removed (already did that)
2. olh entry is marked as 'pending removal'
 - any new versions coming in now to the olh entry will clear pending
   state
3. rgw conditionally removes olh object (still missing condition check)
 - condition checks should verify olh tag & version, and no pending
   modifications
4. if (2) is successful, rgw sends 'clear olh' request to bucket index
5. bucket index, if still 'pending removal' clears olh and plain entry

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw, cls_rgw: conditionally remove olh object

still not completely working, need to debug

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>