Reduce time to write guard-related changes to the state file

We have:

void
entry_guards_changed(void)
{
  time_t when;
  entry_guards_dirty = 1;

  /* or_state_save() will call entry_guards_update_state(). */
  when = get_options()->AvoidDiskWrites ? time(NULL) + 3600 : time(NULL)+600;
  or_state_mark_dirty(get_or_state(), when);
}

Which means that Tor saves guard-related changes to the state file after 10 minutes, or 1 hour if AvoidDiskWrites is enabled.

Maybe this time should be reduced, since we are considering guard-related changes as quite important? It would be a pity to settle on a guard node, then close the Tor client fast and lose that information.

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