This patch doesn't cover every case; omitted cases are marked with
"XXXX prop271", as usual.  It leaves both the old interface and the
new interface for guard status notification, since they don't
actually work in the same way: the new API wants to be told when a
circuit has failed or succeeded, whereas the old API wants to know
when a channel has failed or succeeded.

I ran into some trouble with directory guard stuff, since when we
pick the directory guard, we don't actually have a circuit to
associate it with.  I solved that by allowing guard states to be
associated with directory connections, not just circuits.