cls_rgw: fix rgw_cls_usage_log_trim_op encode/decode

cls_rgw: fix rgw_cls_usage_log_trim_op encode/decode

It was not encoding user, adding that and reset version
compatibility.
This changes affects command interface, makes use of
radosgw-admin usage trim incompatible. Use of old
radosgw-admin usage trim should be avoided, as it may
remove more data than requested. In any case, upgraded
server code will not handle old client's trim requests.

backport: argonaut
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>