Create an executor_work_guard
object.
template< typename ExecutionContext> executor_work_guard< typename ExecutionContext::executor_type > make_work_guard( ExecutionContext & ctx, typename constraint< is_convertible< ExecutionContext &, execution_context & >::value >::type = 0);
An execution context, from which an executor will be obtained.
A work guard constructed with the execution context's executor, obtained
by performing ctx.get_executor()
.