This reverts commit e64883ee
and 40dd2d69

Seems like core.run() returns once its done even if there
are still tasks in the Runtime underneath. A way to solve that
would be to call the shutdown_on_idle method.

We need ownership of the threadpool in order to invoke
`shutdown_on_idle` method but core.runtime only returns a
referrence so we need to create our own threadpool.