protoc-gen-go:generate the code that the field's descriptor map of all message

[zjj9850]

When i use the message's descriptor in golang,but i found the efficiency losing when call descriptor.ForMessage(xx).

I add the code in protoc-gen-go that can generate the descriptor of message when the program init,so the descritpor can be called from memory.the returns is the map structure(the key is the field name and the value is the pointer of the field descriptor)

the code that generated like this:

func GetDbCronJobInfoDescriptorMap() map[string]*protoc_gen_descriptor.FieldDescriptorProto {
 if xxx_descriptor_info_map_DbCronJobInfo == nil {
		initDbCronJobInfoDescriptor()
	}
	return xxx_descriptor_info_map_DbCronJobInfo
}

func initDbCronJobInfoDescriptor() {
	if xxx_descriptor_info_map_DbCronJobInfo == nil {
		xxx_descriptor_info_map_DbCronJobInfo = make(map[string]*protoc_gen_descriptor.FieldDescriptorProto)
		var DbCronJobInfoType *DbCronJobInfo
		_, md := protobuf_descriptor.ForMessage(DbCronJobInfoType)
		for _, fd := range md.Field {
			xxx_descriptor_info_map_DbCronJobInfo[fd.GetName()] = fd
		}
	}
}

var xxx_descriptor_info_map_DbCronJobInfo map[string]*protoc_gen_descriptor.FieldDescriptorProto

func init() {
	proto.RegisterType((*DbCronJobInfo)(nil), "DbCronJobInfo")
	proto.RegisterMapType((map[int32]string)(nil), "DbCronJobInfo.ResultCodeMapEntry")
	initDbCronJobInfoDescriptor()
}

[dsnet]

The v1 descriptor package will be deprecated in the next release and there are no plans to optimize it. The replacement for it is native reflection built into messages. See #364