Benefit over old default is, that we get a short etemplate_exec_id giving better performance for ajax requests.
Drawback is currently poor garbadge collection only removing requests not used in 4 hours or which call egw_framework::window_close on server-side.
We can improve garbadge-collection by binding to window on(before)unload sending a (synchronious) request to server to remove concerned eT2 request.
