Make System::with_tokio_rt take a `FnOnce`

[DzenanJupic]

System::with_tokio_rt currently takes a Fn, which is quite restricting, since it does not allow moving ownership of captured variables into the closure (at least not in a clean way). This is especially useful if the runtime had to be initialized before the system and was stored in a global variable or passed as an argument.

The code in with_tokio_rt only calls the closure once, and since Fn: FnMut and FnMut: FnOnce, this should not be a breaking change.