Clean up Windows graveyard

When a command times out on Windows, we [move](https://github.com/pokey/talon-vscode-command-client/blob/7008cdf83e339220d252fcd32e522eff317a68cf/command_client.py#L230-L239) the request / response file to a graveyard because it is not possible to unlink a file that is in use.  We need some process to clean up these files periodically