Discover new __consumer_offsets partitions

Nobody has claimed this yet.

Perhaps an unlikely use case, but I made the mistake of starting Burrow before running any consumers during some local testing in a small docker-compose environment & lost a lot of time trying to figure out why Burrow wasn't discovering new consumer groups. I _think_ it boils down to this log message:

```
{"level":"info","ts":1516057630.5676234,"msg":"starting consumers","type":"module","coordinator":"consumer","class":"kafka","name":"eventbus","topic":"__consumer_offsets","count":0}
```

Notice the `count` field, which is zero because the __consumer_offsets topic does not exist at the time Burrow starts. After running a consumer to force creation of `__consumer_offsets` & restarting Burrow, it starts detecting new consumer groups as expected. Again, the hint is in the log message:

```
{"level":"info","ts":1516057932.7690308,"msg":"starting consumers","type":"module","coordinator":"consumer","class":"kafka","name":"eventbus","topic":"__consumer_offsets","count":50}
```

I sort of half suspect Burrow might also silently ignore new `__consumer_offsets` partitions even if the topic exists, but but haven't had a chance to check.

Would be nice if we could either flame out hard if no `__consumer_offsets` topic is present or (perhaps ideally) automatically discover any changes to `__consumer_offsets`.

Start by reproducing Burrow startup before Kafka creates the __consumer_offsets topic, using the logged “starting consumers” path as the entry point. Check whether newly created or added __consumer_offsets partitions are discovered after startup; done means Burrow either reliably detects them or clearly fails when the topic is absent.

Written by the indexing model from the issue text.