[Bug] Auto-restart Performers

**Is your feature request related to a problem? Please describe.**

We noticed that a consumer count was going down and not recovering in Rabbit. Restarting a service brings the count back up.

From what we found in code, Brighter knows how to get to required number on start-up, but we could not find anything about some kind of a process that oversees the numbers of performers and gets them back up.

**Describe the solution you'd like**
Have `auto-restart` flag for performers, so they reconnect in case they disappear. It would also be nice to know why a performer disappeared.

**Describe alternatives you've considered**
Restarting services. but gets a bit tedious.

**Additional context**
Add any other context or screenshots about the feature request here.

Ian's comments on the issue from gitter:

```
When we lose a performer we don't auto-restart it. We could. Probably we want to make that configurable i.e. auto-restart and we would need to make sure that we were not in shutdown when we did that.
I am looking at message pumps (kick @DevJonny for me) and I could add that feature pretty fast, if you guys wanted to raise an issue
```