Deploy rdsys alongside BridgeDB

Once we have an MVP, it's time to deploy rdsys on polyanthum, alongside BridgeDB. An MVP could (re)implement the HTTPS distributor (#2 (closed)) and/or Salmon (#1 (closed)). Here's how BridgeDB and rdsys could share bridge descriptors:

1. BridgeDB and rdsys could both pretend that the other one doesn't exist and simply use all descriptors. That's the simplest solution but it increases the odds of a bridge being blocked (because it's handed out by more than one distributor)

2. We could siphon off a fraction of our descriptors and reserve it for rdsys. That means that both rdsys and BridgeDB need code that decides who a given descriptor belongs to.

Milestone for this in http://eweiibe6tdjsdprb4px6rqrzzcsi22m4koia44kc5pcjr7nec2rlxyad.onion/tpo/anti-censorship/rdsys/-/milestones/1