XL: Introduce new API StorageInfo. (#1770)

This is necessary for calculating the total storage
capacity from object layer. This value is also needed for
browser UI.

Buckets used to carry this information, this patch
deprecates this feature.

object-interface.go

@@ -20,6 +20,9 @@ import "io"
 
 // ObjectLayer implements primitives for object API layer.
 type ObjectLayer interface {
+	// Storage operations.
+	StorageInfo() StorageInfo
+
 	// Bucket operations.
 	MakeBucket(bucket string) error
 	GetBucketInfo(bucket string) (bucketInfo BucketInfo, err error)