Make `zb serve` more resilient for restarts

[zombiezen]

Currently we exit if ZB_STORE_SOCKET already exists. Ideally, the socket would always get cleaned up, but especially on macOS/Windows, where we're managing the socket ourselves, this can get messy if there's an unclean shutdown. Having the user have to delete the file is really annoying and confusing, so leaving an issue here to eliminate this papercut.