Only fetch microdescriptors from 0.2.3.1-alpha and later (and authorities)
Right now in master, we fetch microdescriptors from authorities and from any host that is running 0.2.3.0-alpha (i.e, the code from git before the 0.2.3.1-alpha release) or later. That's because there are currently (in the consensus I see) 11 hosts running 0.2.3.0-alpha-dev and none running 0.2.3.1 yet. But at some point soon, we should switch the code so that 0.2.3.1-alpha is required.
The tradeoff here is that 0.2.3.0-alpha-dev hosts are likelier to be broken, but 0.2.3.1-alpha hosts are currently nonexistent.