rgw: rgw-rados, rgw-admin add an option to recalculate user stats

rgw: rgw-rados, rgw-admin add an option to recalculate user stats

Adds a method in rgw-rados to reset user stats calling the earlier implemented
cls user reset stats.
In rgw-admin we add an option called --reset-stats that invokes this method.

Fixes: http://tracker.ceph.com/issues/23335
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 0b72dd7d3beea47b5c479eeb979b73c4cd70f7bc)

 Conflicts:
  src/rgw/rgw_admin.cc
  whitespace conflict
  src/rgw_rados.cc
  switch to jewel version of get_obj_ref (where raw version isn't available)

jewel only admin fixes