Allow CloseBucket behavior on IBucket in addition to ICluster

The ICluster interface has the CloseBucket() method, which is the only way to cleanly release bucket resources. If the bucket is opened and used for an extended time (eg. Repository pattern), instances of both the Cluster and Bucket have to be held, though the only reason to keep the Cluster around is to eventually close the bucket. If the Bucket had it's own Close() method, only one handle would be required. This assumes there isn't some other reason the Cluster needs to be held open.