ZipVirtualPathProvider takes an exclusive lock on the backing zip file

Steps to reproduce

When the ZipVirtualPathProvider is used for serving files from within a zip archive, the backing zip file is locked exclusively.

This prevents automatic deployment of sites without restarting the application pool. Restarting the application pool shouldn't be necessary when releasing new code.

1. Make sure the site is running by loading the CMS UI in a browser

2. Try to delete or replace modules/_protected/EPiServer.Shell.UI.zip

The file can't be deleted since it's locked by an IIS worker process.