Hidden services should try harder to reuse their old intro points

The current hidden service behavior is that when Tor loses its intro circuits, it chooses new intro points and makes new circuits to them -- which means anybody who has the old hidden service descriptor is going to be introducing herself to the wrong intro points.

If our intro circuits close, but it was because our network failed and not because the intro points failed, we should reestablish new intro circuits to the old intro points.

Nathan wants this for running hidden services on Orbot, since Orbot users change networks (and thus lose existing circuits) quite often.

I expect the main tricky point to be distinguishing "network failed" from "intro point failed". I wonder if some of our CBT work is reusable here.