[#26] object: Replace CreationPoint with Epoch

In previous version of the format Object message included moment of creation
in two epoch times: NeoFS and Unix. Since unix  is not processed in the
system, this commit narrows the time point of object creation to the system
epoch number. The object.CreationPoint message has therefore been deleted
as no longer used

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

object/types.proto

@@ -57,15 +57,8 @@ message SystemHeader {
     bytes OwnerID = 4;
     // CID is a SHA256 hash of the container structure (container identifier)
     bytes CID = 5;
-    // CreatedAt is a timestamp of object creation
-    CreationPoint CreatedAt = 6;
-}
-
-message CreationPoint {
-    // UnixTime is a date of creation in unixtime format
-    int64 UnixTime = 1;
-    // Epoch is a date of creation in NeoFS epochs
-    uint64 Epoch   = 2;
+    // CreationEpoch carries number of NeoFS epoch on which the object was created.
+    uint64 CreationEpoch = 6;
 }
 
 message IntegrityHeader {

proto-docs/object.md

@@ -30,7 +30,6 @@
 
   - Messages
     - [Attribute](#object.Attribute)
-    - [CreationPoint](#object.CreationPoint)
     - [ExtendedHeader](#object.ExtendedHeader)
     - [Header](#object.Header)
     - [IntegrityHeader](#object.IntegrityHeader)
@@ -374,18 +373,6 @@ Attribute groups the parameters of the object attributes.
 | Value | [string](#string) |  | Value carries the string value of the object attribute. |
 
 
-<a name="object.CreationPoint"></a>
-
-### Message CreationPoint
-
-
-
-| Field | Type | Label | Description |
-| ----- | ---- | ----- | ----------- |
-| UnixTime | [int64](#int64) |  | UnixTime is a date of creation in unixtime format |
-| Epoch | [uint64](#uint64) |  | Epoch is a date of creation in NeoFS epochs |
-
-
 <a name="object.ExtendedHeader"></a>
 
 ### Message ExtendedHeader
@@ -479,7 +466,7 @@ Header groups the information about the NeoFS object.
 | ID | [bytes](#bytes) |  | ID is an object identifier, is a valid UUIDv4 |
 | OwnerID | [bytes](#bytes) |  | OwnerID is a wallet address |
 | CID | [bytes](#bytes) |  | CID is a SHA256 hash of the container structure (container identifier) |
-| CreatedAt | [CreationPoint](#object.CreationPoint) |  | CreatedAt is a timestamp of object creation |
+| CreationEpoch | [uint64](#uint64) |  | CreationEpoch carries number of NeoFS epoch on which the object was created. |
 
 
 <a name="object.Tombstone"></a>