Expose we_fetch_{micro,router_}descriptors on control port
Right now, stem has some slightly wrong logic:
https://gitweb.torproject.org/stem.git/tree/stem/control.py#n1828
We should make GETINFO commands to fetch these. How about
```
info/config/we-download-microdescs and
info/config/we-download-routerdescs
```
?
issue