serve s3: fixes before merge

- add context to log and fallthrough to error log level
- test: use rclone random lib to generate random strings
- calculate hash from vfs cache if file is uploading
- add server started log with server url
- remove md5 hasher

cmd/serve/s3/s3.go

@@ -61,7 +61,10 @@ var Command = &cobra.Command{
 			}
 			router := s.Router()
 			s.Bind(router)
-			s.Serve()
+			err = s.serve()
+			if err != nil {
+				return err
+			}
 			s.Wait()
 			return nil
 		})