rgw: use multiple notification objects

rgw: use multiple notification objects

Issue #2504. This makes us listen and notify on more than
a single object, which reduces the contention of cache
notifications.

NOTE: This change requires that any radosgw and radosgw-admin
use the same 'rgw num control oids' config value. A config value
of 0 will maintain old compatibility, and will allow an upgraded
process run in conjuction with an old one. Setting value other
than 0 (or using the non-zero default) will require upgrading
and restarting all the gateways together. Failing to do so
might lead to inconsistent user and buckets metadata (which
will be resolved once gateways are restarted).

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