v2: Define API service interfaces

Define interfaces that reflect service (Accounting, Container, Session)
declarations in the API.

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

v2/session/types.go

@@ -11,28 +11,12 @@ type CreateRequestBody struct {
 	expiration uint64
 }
 
-type CreateRequest struct {
-	body *CreateRequestBody
-
-	metaHeader *RequestMetaHeader
-
-	verifyHeader *RequestVerificationHeader
-}
-
 type CreateResponseBody struct {
 	id []byte
 
 	sessionKey []byte
 }
 
-type CreateResponse struct {
-	body *CreateResponseBody
-
-	metaHeader *ResponseMetaHeader
-
-	verifyHeader *ResponseVerificationHeader
-}
-
 type XHeader struct {
 	key, val string
 }