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