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/metacache-set.go

@@ -540,10 +540,6 @@ func (er *erasureObjects) streamMetadataParts(ctx context.Context, o listPathOpt
 	}
 }
 
-func (er erasureObjects) SetDriveCount() int {
-	return er.setDriveCount
-}
-
 // Will return io.EOF if continuing would not yield more results.
 func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entries metaCacheEntriesSorted, err error) {
 	o.debugf(color.Green("listPath:")+" with options: %#v", o)
@@ -598,9 +594,9 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 	askDisks := o.AskDisks
 	listingQuorum := askDisks - 1
 	// Special case: ask all disks if the drive count is 4
-	if askDisks == -1 || er.SetDriveCount() == 4 {
+	if askDisks == -1 || er.setDriveCount == 4 {
 		askDisks = len(disks) // with 'strict' quorum list on all online disks.
-		listingQuorum = getReadQuorum(er.SetDriveCount())
+		listingQuorum = getReadQuorum(er.setDriveCount)
 	}
 
 	if len(disks) < askDisks {