rgw: stream list buckets (containers) request

Fixes: #4760
Instead of retrieving the entire list of buckets in one
chunk, streamline it. This makes it so that if the request
takes too long, client isn't going to timeout before getting
any data.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: list container only shows stats if needed

Fixes: #4759
Add a new request param 'stats' for the swift list containers
request. If set to 'false' it disables stats retrieval, which
makes it go faster. Also, don't dump stats if format is plain,
as they're not going to be dumped.

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