fix hang in `./openbazaar stop`

Sometimes `./openbazaar stop` fails to stop the server and ends up hanging itself too.

Recently it seems we have picked up a crash that once again requires sigkill. I think sigkill never did work with the new stop function maybe.

Currently, if sigterm doesn't work and `psutil.wait_procs()` is called, it never returns. We don't need to call that anyway.

Further testing is warranted. I'm posting this now because I don't want to lose track of it while I'm working on other things.