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-datatypes.go

@@ -18,12 +18,16 @@ package main
 
 import "time"
 
+// StorageInfo - represents total capacity of underlying storage.
+type StorageInfo struct {
+	Total int64 // Total disk space.
+	Free  int64 // Free total available disk space.
+}
+
 // BucketInfo - bucket name and create date
 type BucketInfo struct {
 	Name    string
 	Created time.Time
-	Total   int64
-	Free    int64
 }
 
 // ObjectInfo - object info.