Avoid needless router_dir_info_has_changed from router_set_status

On some profiles of Andrea's from #11332, I found that a great deal
of time can still be attributed to functions called from
update_router_have_minimum_dir_info().  This is making our
digestmap, tor_memeq, and siphash functions take a much bigger
portion of runtime than they really should.

If we're calling update_router_have_minimum_dir_info() too often,
that's because we're calling router_dir_info_changed() too often.
And it looks like most of the callers of router_dir_info_changed()
are coming as tail-calls from router_set_status() as invoked by
channel_do_open_actions().

But we don't need to call router_dir_info_changed() so much!  (I'm
not quite sure we need to call it from here at all, but...) Surely
we don't need to call it from router_set_status when the router's
status has not actually changed.

This patch makes us call router_dir_info_changed() from
router_set_status only when we are changing the router's status.

Fix for bug 12170.  This is leftover from our fix back in 273ee3e81
in 0.1.2.1-alpha, where we started caching the value of
update_router_have_minimum_dir_info().
Merge remote-tracking branch 'public/bug12170_024_v2'