Don't delay descriptor fetches when missing info needed for circuits

Don't delay descriptor fetches when missing info needed for circuits

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.