Make sure to close the disk connections (#5752)

Since we do not re-use storageDisks after moving
the connections to object layer we should close them
appropriately otherwise we have a lot of connection
leaks and these can compound as the time goes by.

This PR also refactors the initialization code to
re-use storageDisks for given set of endpoints until
we have confirmed a valid reference format.

cmd/xl-v1.go

@@ -59,14 +59,7 @@ var xlTreeWalkIgnoredErrs = append(baseIgnoredErrs, errDiskAccessDenied, errVolu
 // Shutdown function for object storage interface.
 func (xl xlObjects) Shutdown(ctx context.Context) error {
 	// Add any object layer shutdown activities here.
-	for _, disk := range xl.getDisks() {
-		// This closes storage rpc client connections if any.
-		// Otherwise this is a no-op.
-		if disk == nil {
-			continue
-		}
-		disk.Close()
-	}
+	closeStorageDisks(xl.getDisks())
 	return nil
 }