When we have fewer than 15 descriptors to fetch, we will delay the
fetch for a little while. That's fine, if we can go ahead and build
circuits... but if not, it's a poor choice indeed.
Fixes bug 23985; bugfix on 0.1.1.11-alpha.
In 0.3.0.3-alpha, when we made primary guard descriptors necessary
for circuit building, this situation got worse.
