validate storage class across pools when setting config (#11320)

```
mc admin config set alias/ storage_class standard=EC:3
```

should only succeed if parity ratio is valid for all
server pools, if not we should fail proactively.

This PR also needs to bring other changes now that
we need to cater for variadic drive counts per pool.

Bonus fixes also various bugs reproduced with

- GetObjectWithPartNumber()
- CopyObjectPartWithOffsets()
- CopyObjectWithMetadata()
- PutObjectPart,PutObject with truncated streams

cmd/api-headers.go

@@ -156,16 +156,16 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, rs *HTTPRangeSp
 		return err
 	}
 
-	if opts.PartNumber > 0 {
-		rs = partNumberToRangeSpec(objInfo, opts.PartNumber)
-	}
-
 	// For providing ranged content
 	start, rangeLen, err = rs.GetOffsetLength(totalObjectSize)
 	if err != nil {
 		return err
 	}
 
+	if rs == nil && opts.PartNumber > 0 {
+		rs = partNumberToRangeSpec(objInfo, opts.PartNumber)
+	}
+
 	// Set content length.
 	w.Header().Set(xhttp.ContentLength, strconv.FormatInt(rangeLen, 10))
 	if rs != nil {