Removing discard Future and asyncspawn

[lchenut]

Discarding Future is generally a bad idea because the Future created could be "garbage collected" and cause segfault with a wrong condition.

Even if it's a better way of doing things, asyncspawn is also not recommended because it's uncontrollable. It creates a Future artifact to which nobody has access.

For both this problems, it's best to store the resulting Future in some object so that we can, at the very least, cancel it.