[nspcc-dev#194] object: Define payload format of LOCK objects

Define `object.Lock` message which carries list of locked objects.
Require this message to be a payload of locked objects (`LOCK` type).

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

object/types.proto

@@ -177,6 +177,14 @@ message Header {
 // Object structure. Object is immutable and content-addressed. It means
 // `ObjectID` will change if header or payload changes. It's calculated as a
 // hash of header field, which contains hash of object's payload.
+//
+// Payload format depends on object type specified in the header:
+// * **LOCK** \
+// Must be encoded message of type `Lock` in Protocol Buffers binary format
+// with direct field order. Member list must no be empty or carry empty IDs.
+// All members must regular objects (`REGULAR` type). Lifetime of the lock
+// object is limited similar to regular objects in `__NEOFS__EXPIRATION_EPOCH`
+// attribute.
 message Object {
   // Object's unique identifier.
   neo.fs.v2.refs.ObjectID object_id = 1 [json_name = "objectID"];
@@ -208,3 +216,9 @@ message SplitInfo {
   // object parts.
   neo.fs.v2.refs.ObjectID link = 3;
 }
+
+// Lock keeps record of objects that are locked.
+message Lock {
+  // List of objects to be locked.
+  repeated neo.fs.v2.refs.ObjectID members = 1 [json_name = "members"];
+}