Fix race condition in shutdown behavior

[bochen87]

The behaviour we're observing with the ThreadedSocketCANInterface is similar to what is described here: https://stackoverflow.com/questions/68787876/boostasio-server-code-causes-segfault-only-some-of-the-time-seemingly-relat

This seems to fix those segfaults happening at shutdown. Somehow it only works though if I put it into the dtor, not if I put it into the shutdown_internal function - then it's basically not possible to shut it down anymore.