bucket: refactor policies and fix bugs related to enforcing policies. (#2766)

This patch also addresses the problem of double caching at
object layer once at XL and another at handler layer.

cmd/storage-errors.go

@@ -59,6 +59,3 @@ var errVolumeAccessDenied = errors.New("volume access denied")
 
 // errVolumeAccessDenied - cannot access file, insufficient permissions.
 var errFileAccessDenied = errors.New("file access denied")
-
-// errVolumeBusy - remote disk is not connected to yet.
-var errVolumeBusy = errors.New("volume is busy")