fix: make complete multipart uploads faster encrypted/compressed backends (#15375)

- Only fetch the parts we need and abort as soon as one is missing.
- Only fetch the number of parts requested by "ListObjectParts".

cmd/storage-datatypes.go

@@ -285,6 +285,7 @@ type ReadMultipleReq struct {
 	MaxSize      int64    // Return error if size is exceed.
 	MetadataOnly bool     // Read as XL meta and truncate data.
 	AbortOn404   bool     // Stop reading after first file not found.
+	MaxResults   int      // Stop after this many successful results. <= 0 means all.
 }
 
 // ReadMultipleResp contains a single response from a ReadMultipleReq.