rgw: add bucket size limit check to radosgw-admin

rgw: add bucket size limit check to radosgw-admin

The change adds a new list of all buckets x all users, with
fields for bucket name, tenant name, current num_objects,
current num_shards, current objects per shard, and the
corresponding fill_status--the latter consisting of 'OK',
'WARN <n>%', or 'OVER <n>%.'

The warning check is relative to two new tunables.  The threshold
max objects per shard is set as rgw_bucket_safe_max_objects_per_shard,
which defaults to 100K.  The value rgw_bucket_warning_threshold is
a percent of the current safe max at which to warn (defaults to
90% of full).

From review:

* fix indentation (rgw_admin)
* if user a user_id is provided, check only buckets for that user
* update shard warn pct to be pct-of-fill (not 100 - pct-of-fill)
* print only buckets near or over per-shard limit, if --warnings-only
* s/bucket limitcheck/bucket limit check */
* sanity shard limit should be 90, not 10 (because that changed)
* fixes for memleaks and other points found by cbodley

Fixes: http://tracker.ceph.com/issues/17925
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>