In ConcurrentSetSpanAndGetSpan, a writer thread creates new spans until the readers are done with 10000 reads.

    stdx::thread writer([&] {
        while (!stop.load()) {
            impl.setSpan(makeValidSpan());
        }
    }); 

Every setSpan call appends another DataList node, so on machines where the reads take a while, this causes stack overflow.

We should either cap the number of writes here, or just reuse a valid span.