Update todo

todo.md

@@ -1,7 +1,5 @@
 - Not working behind Nginx
 - Try sending the http_response "OK" _after_ the request body is received
-- Add relay / server debug flags in config
-- Tracing / structured logging, maybe?
 
 - Sort directory listings
 - ".." from server to server list is broken
@@ -11,7 +9,7 @@
 - Make file_server_root mandatory
 - Deny unused HTTP methods for endpoints
 - Hide ptth_server.toml from file server
-- ETag cache
+- ETag cache based on mtime
 - Server-side hash?
 - Log / audit log?