spec: Update recommendations for DBUS_COOKIE_SHA1 timeouts
This had two issues that could damage interoperability.
First, the spec wording suggested that any cookie that had not been deleted was suitable for use in authentication. However, this introduces a race condition, which is called out in comments in both the reference implementation and GDBus: the newest cookie might be less old than the arbitrary lifetime when authentication begins, but older than the lifetime at the time authentication ends. As a result, we need a grace period during which an old cookie will still be accepted, but a newer cookie exists and will be used for new authentication operations.
Second, the spec wording implied that the arbitrary timeouts were completely up to the implementor. However, GLib bug https://gitlab.gnome.org/GNOME/glib/-/issues/2164 indicates that they need to be reasonably compatible: in particular, GDBus servers historically didn't allocate new cookies until 10 minutes had passed, but libdbus clients would decline to use a cookie older than 5 minutes, causing authentication to fail if the gdbus-server test-case (in which GDBus and libdbus clients connect to a GDBus server) happened to take longer than 5 minutes to run.
While I'm here, also be consistent about calling the secrets "cookies" (consistent with the name of the mechanism) rather than "keys" (which is what they are called in libdbus' dbus-keyring.c).