Fixing #2325

[sithhell]

Fixes #2325

Proposed Changes

As time went by, we can re-enable the usage of executors again. Some changes
in our code base seem to have led to this not segfaulting anymore

[hkaiser]

That's the worst of all - they just stand up and disappear - how antisocial ;-)

[taeguk]

GOOD! maybe you want to refer #2325 instead of #2556 in title text and commit message.

[taeguk]

hpx/hpx/parallel/util/scan_partitioner.hpp

Lines 128 to 144 in 3b0ecd7

	hpx::launch p = hpx::launch::sync;
	// if (parts & 0x7)
	// p = hpx::launch::sync;
	FwdIter it = hpx::util::get<0>(elem);
	std::size_t size = hpx::util::get<1>(elem);
	hpx::shared_future<Result1> prev = workitems.back();
	auto curr = executor_traits::async_execute(
	policy.executor(), f1, it, size).share();
	workitems.push_back(dataflow(p, f2, prev, curr));
	finalitems.push_back(dataflow(p, //olicy.executor(),
	f3, it, size, prev, curr));
	++parts;

if (parts & 0x7) 
    p = hpx::launch::sync;

In past days, there was the code in scan_partitioner.
It disappeared in #2556 even in the comments.
And this PR didn't apply it.

[hkaiser]

@taeguk I think this was an attempt to avoid spawning asynchronous tasks for every partition, the tasks spawned tend to be very small.