OtrlMessageType — the possible types of internal messages transmitted
These values are returned by otrl_proto_message_type.
OTRL_MSGTYPE_NOTOTR OTRL_MSGTYPE_TAGGEDPLAINTEXT OTRL_MSGTYPE_QUERY OTRL_MSGTYPE_DH_COMMIT OTRL_MSGTYPE_DH_KEY OTRL_MSGTYPE_REVEALSIG OTRL_MSGTYPE_SIGNATURE OTRL_MSGTYPE_V1_KEYEXCH OTRL_MSGTYPE_DATA OTRL_MSGTYPE_ERROR OTRL_MSGTYPE_UNKNOWN