Panic on start when setting affinity

[Stebalien]

I have a user report about minstant panicing on start when it calls set_affinity:

https://github.com/tikv/minstant/blob/27c9ec5ec90b5b67113a748a4defee0d2519518c/src/tsc_now.rs#L112

I'm still trying to get to the bottom of what's causing this issue, but it would be nice to just return TSCLevel::Unstable on error instead.