Clean up session files in compile run
During the compile run, certain / most caches are rebuilt. If an account is currently logged in, it won't be able to access restricted resources after a compile run because the session cookie doesn't have corresponding information in FLOW3 anymore. In this case the user will experience an Access Denied exception (not a nice experience).
If the PHP session data would be trashed during the compile run, the user would get a fresh session cookie on the next hit and instead of the Access Denied would be redirected to an authentication entry point due to a AuthenticationRequiredException.