[C++] Use FnOnce for TaskGroup's tasks instead of std::function

[asfimport]

FnOnce drops dependencies on invocation and is lighter weight than std::function

Reporter: Michal Nowakiewicz / @michalursa
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Let Futures support cancellation (is superceded by)

_{Note: This issue was originally created as ARROW-11191. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
In addition, I think we no longer need distinct TaskGroup::Append/AppendReal or ThreadPool/Executor::Submit/SubmitReal, I think the protected *Real functions could be deleted.

[asfimport]

Antoine Pitrou / @pitrou:
This was done as part of ARROW-8732.