Fix a problem about asynchronous execution of parallel::merge and parallel::partition.

[taeguk]

No description provided.

[sithhell]

Could we prefect forward comp, proj1 and proj2 into the lambda?

[taeguk]

@sithhell I did. There was 3 key points.

1. The lambda must have own variables including comp, proj1, and proj2 because the variables will be used in asynchronous manner.
2. So, the lambda must construct own variables from original parameters. In this point, the parameters must be perfect forwarded into constructors of the own variables.
3. Support C++11. (This is why I use std::bind().)

[taeguk]

@sithhell How do you think about my modification for perfect forwarding?
There are some places that this way can be applied to. (For example,

hpx/hpx/parallel/util/scan_partitioner.hpp

Lines 342 to 350 in a5ac8fc

	hpx::future<R> f = execution::async_execute(
	policy.executor(),
	[=]() mutable -> R
	{
	return static_scan_partitioner_helper<
	R, Result1, Result2, ScanPartTag
	>::call(policy, first, count, init,
	f1, f2, f3, f4);
	});

)

If this way is appropriate enough, I will apply it to some places, too.

[hkaiser]

LGTM, thanks a lot!