Limit tokio to current thread

[Firstyear]

Hi there,

In https://github.com/pendulum-project/ntpd-rs/blob/main/ntpd/bin/ntp-daemon.rs#L5 you are using tokio::main which activates the multi-thread scheduler. On my xeon server with 24 threads, this means that I have ntpd-rs with 24 idle threads.

I would assume that setting tokio to the current_thread runtime flavour would probably be okay for the project, and also seems like a simpler/less resource consuming default?