[#250] v2/container: Add epoch field to size announce body

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

v2/container/types.go

@@ -94,6 +94,8 @@ type GetExtendedACLResponseBody struct {
 }
 
 type UsedSpaceAnnouncement struct {
+	epoch uint64
+
 	cid *refs.ContainerID
 
 	usedSpace uint64
@@ -917,6 +919,20 @@ func (r *GetExtendedACLResponse) SetVerificationHeader(v *session.ResponseVerifi
 	}
 }
 
+func (a *UsedSpaceAnnouncement) GetEpoch() uint64 {
+	if a != nil {
+		return a.epoch
+	}
+
+	return 0
+}
+
+func (a *UsedSpaceAnnouncement) SetEpoch(v uint64) {
+	if a != nil {
+		a.epoch = v
+	}
+}
+
 func (a *UsedSpaceAnnouncement) GetUsedSpace() uint64 {
 	if a != nil {
 		return a.usedSpace