double ratchet first implementation

pb/messages.proto

@@ -138,6 +138,8 @@ message ContactCard {
   uint32              version               = 7;
   string              invitation_id         = 8;
   string              invitation_message    = 9;
+  string              dr_root_key           = 10; // DR pre-shared root key (base64, 32 bytes)
+  string              dr_public_key         = 11; // DR DH public key of the initiator (base64)
 }
 
 // structure for sending a message to be forwarded to another user in protobuf format
@@ -147,6 +149,7 @@ message PackedUserMessage {
   bytes          signature            = 3; // the payload signature with the client identity private key
   repeated int64 server_timestamp     = 4; // server time stamp, might be several in matriochka mode
   string         server_delivery_uuid = 5; // message uuid, for server delivery tracking, omitted if not delivery tracking desired
+  bytes          dr_header            = 6; // serialized doubleratchet MessageHeader; empty = no DR layer
 }
 
 message ConversationStatus {