Fix busy-loop when consensus is near expiring.

When our current consensus is getting close to being invalid (but it isn't invalid yet), we try to get a new one. So far, so good.

But we had a bug: when we went to get a new consensus, we'd see that we had a perfectly fine not-yet-invalid consensus in our cache, reload it, find that it was ready, and continue!

This patch fixes our behavior: If we have a usable consensus, then when we reset the bootstrapping process, we ignore any cached consensus.

Fixes #274 (closed).