The current code uses approximation using a ratio. The approximation
can skew if we have multiple pools with different disk capacities.
Replace the algorithm with a simpler one which counts data
disks and ignore parity disks.
```
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
