Stop BridgeDB from handing out offline bridges

BridgeDB currently hands out plenty of bridges (in all flavours) that are offline. We need to understand why this is the case, and stop it from doing that.

For example, I just got the obfs4 bridge 4C480695650EDB6BAB006DB9FD81F6173122E973 over HTTPS. Nothing responds on its obfs4 port and Metrics says that it's currently offline -- or used to be, a few hours ago, to be precise. The bridge's IP address is part of Serge's most recent networkstatus-bridges file, but the bridge does not have the Running flag and should not have been given out. Also, the bridge's fingerprint isn't part of BridgeDB's latest assignments.log file. According to all of this, I should not have been given that bridge.