Classify chunk_size_iterator to input iterator tag. (Fix #2866)

[taeguk]

This PR fixes #2866 .

Now, I classify chunk_size_iterator to input_iterator_tag, not forward_iterator_tag.
What I worry is that if chunk_size_iterator will be used incorrectly in any place, he/she may not know what is incorrect and wander with mistery bug.

hpx/hpx/parallel/executors/execution_fwd.hpp

Lines 193 to 201 in 954818a

	template <typename Executor, typename F, typename Shape, typename ... Ts>
	HPX_FORCEINLINE auto
	bulk_async_execute(Executor && exec, F && f, Shape const& shape,
	Ts &&... ts)
	-> decltype(bulk_async_execute_fn_helper<
	typename std::decay<Executor>::type
	>::call(std::forward<Executor>(exec), std::forward<F>(f),
	shape, std::forward<Ts>(ts)...
	))

For example, in above code, there is a nothing which explicitly notices that chunk_size_iterator is input iterator.
So, an user who utilizes chunk_size_iterator "indirectly" can make a mistake which violates multi-pass gaurantee.

[hkaiser]

LGTM, thanks!