Previously we tolerated up to 1.5 connections for every relay we
were connected to, and didn't warn if we had fewer than 5
connections total.

Now we tolerate up to 1.5 connections per relay, and up to 4
connections per authority, and we don't warn at all when we have
fewer than 25 connections total.

Fixes bug 33880, which seems to have been provoked by our #17592
change in 0.3.5.