radosgw-admin: new 'bucket rewrite' command

Iterates through objects and rewrites them.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
radosgw-admin: add some conditions for bucket rewrite

 --min-rewrite-size, --max-rewrite-size to specify object size
 conditions

 --start-date, --end-date to specify object mtime conditions (format is
 YYYY-MM-DD[ HH:MM:SS])

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: don't log system requests in usage log

Fixes: 6889
System requets should not be logged in the usage log.

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