[#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/data/tree.go

@@ -18,6 +18,7 @@ type NodeVersion struct {
 	BaseNodeVersion
 	DeleteMarker  *DeleteMarkerInfo
 	IsUnversioned bool
+	IsCombined    bool
 }
 
 func (v NodeVersion) IsDeleteMarker() bool {
@@ -79,13 +80,13 @@ type MultipartInfo struct {
 
 // PartInfo is upload information about part.
 type PartInfo struct {
-	Key      string
-	UploadID string
-	Number   int
-	OID      oid.ID
-	Size     uint64
-	ETag     string
-	Created  time.Time
+	Key      string    `json:"key"`
+	UploadID string    `json:"uploadId"`
+	Number   int       `json:"number"`
+	OID      oid.ID    `json:"oid"`
+	Size     uint64    `json:"size"`
+	ETag     string    `json:"etag"`
+	Created  time.Time `json:"created"`
 }
 
 // ToHeaderString form short part representation to use in S3-Completed-Parts header.