fs: Break fs package to top-level and introduce ObjectAPI interface.
ObjectAPI interface brings in changes needed for XL ObjectAPI layer.
The new interface for any ObjectAPI layer is as below
```
// ObjectAPI interface.
type ObjectAPI interface {
// Bucket resource API.
DeleteBucket(bucket string) *probe.Error
ListBuckets() ([]BucketInfo, *probe.Error)
MakeBucket(bucket string) *probe.Error
GetBucketInfo(bucket string) (BucketInfo, *probe.Error)
// Bucket query API.
ListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsResult, *probe.Error)
ListMultipartUploads(bucket string, resources BucketMultipartResourcesMetadata) (BucketMultipartResourcesMetadata, *probe.Error)
// Object resource API.
GetObject(bucket, object string, startOffset int64) (io.ReadCloser, *probe.Error)
GetObjectInfo(bucket, object string) (ObjectInfo, *probe.Error)
PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string) (ObjectInfo, *probe.Error)
DeleteObject(bucket, object string) *probe.Error
// Object query API.
NewMultipartUpload(bucket, object string) (string, *probe.Error)
PutObjectPart(bucket, object, uploadID string, partID int, size int64, data io.Reader, md5Hex string) (string, *probe.Error)
ListObjectParts(bucket, object string, resources ObjectResourcesMetadata) (ObjectResourcesMetadata, *probe.Error)
CompleteMultipartUpload(bucket string, object string, uploadID string, parts []CompletePart) (ObjectInfo, *probe.Error)
AbortMultipartUpload(bucket, object, uploadID string) *probe.Error
}
```
Harshavardhana
efc80343e3