[#275] Change logic delete multipart upload

In order not to accidentally take outdated
information about downloaded parts from other
nodes, now when the multipart is abort or complete,
the root node of the multipart upload with the
finish flag remains in the tree.

Signed-off-by: Roman Loginov <r.loginov@yadro.com>

api/data/tree.go

@@ -77,6 +77,7 @@ type MultipartInfo struct {
 	Created       time.Time
 	Meta          map[string]string
 	CopiesNumbers []uint32
+	Finished      bool
 }
 
 // PartInfo is upload information about part.