CBES-5: add _async_load_bucket for single bucket

Change-Id: Ie3a51a8f45b5cd5b41e0e571b4d3d29959733ac7
Reviewed-on: http://review.couchbase.org/23829
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: buildbot <build@couchbase.com>