[#63] Add fast multipart upload

Add new flag to object tree meta `isCombined` that means
the object payload is list of parts that forms real payload.
Set this attribute when complete multipart upload not to do unnecessary copying.

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/layer/layer.go

@@ -111,6 +111,15 @@ type (
 		CopiesNumbers []uint32
 	}
 
+	PutCombinedObjectParams struct {
+		BktInfo    *data.BucketInfo
+		Object     string
+		Size       uint64
+		Header     map[string]string
+		Lock       *data.ObjectLock
+		Encryption encryption.Params
+	}
+
 	DeleteObjectParams struct {
 		BktInfo  *data.BucketInfo
 		Objects  []*VersionedObject
@@ -410,7 +419,7 @@ func (n *layer) ListBuckets(ctx context.Context) ([]*data.BucketInfo, error) {
 func (n *layer) GetObject(ctx context.Context, p *GetObjectParams) (*ObjectPayload, error) {
 	var params getParams
 
-	params.oid = p.ObjectInfo.ID
+	params.objInfo = p.ObjectInfo
 	params.bktInfo = p.BucketInfo
 
 	var decReader *encryption.Decrypter