[concurrency] strands

boost.asio has this neat concept of "strands". (compare https://www.boost.org/doc/libs/1_78_0/doc/html/boost_asio/reference/strand.html)

they basically function as "executor" inside a thread pool, which ensure sequential execution of all work which is dispatched in this strand. it might be worth having a similar concept in QtConcurrent.

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`QtConcurrent::run()` currently uses a `QThreadPool` as argument. the semantics of "strand" is equivalent to using `QtConcurrent::run` with a `QThreadPool` that only has a single worker thread. this is perfectly fine of course, but it requires users to spin up a custom thread and one cannot re-use existing thread pools.

so i could imagine something like:

QtConcurrent::strand myStrand(QThreadPool::globalInstance()); // wrap global threadpool

for (int index : {1, 2, 3, 4}) {
    QtConcurrent::run(myStrand, [index] {
        profit(index); // called sequentially on the global thread pool
    );
}

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one could achieve the same by chaining futures, but that's not always practical to do