We're missing descriptors for some of our primary entry guards

Seems like there are still a few cases where bootstrapping will stall because of:

I learned some more directory information, but not enough to build a circuit: We're missing descriptors for some of our primary entry guards 

It seems to happen mainly with bridges. For me it occured with latest master (b081a7ed2) when I was switching between using private bridges and the default TBB bridges. I also heard reports from s7r about this happening.

We last changed this behavior in #21415 (moved), when we made Tor require that we have the descriptor of 2 bridges before we start building circuits.