tickets/DM-199: Develop new master-worker result system

Member variables that do not appear in the constructor initializer list are default initialized, which for built-in types means they have indeterminate value. For example, the following program prints random integers on my system:

```c++
#include <string>
#include <iostream>

struct A {
    int x;
    std::string foo;
    A(std::string const& s) : foo(s) {}
};

int main() {
    A a("foo");
    std::cout << a.x << std::endl;
}
```
So omitting `_dirty(false)` is a bug. Note that `qdisp::QueryReceiver::Error` suffers from the same problem for its `code` member.