praefect/info: Plug Goroutine leaks in consistencycheck tests

We have two Goroutine leaks in the consistencycheck tests caused by gRPC connections which are never closed. Plug these leaks by closing the connections.

There's one additional Goroutine which is the Rubyserver balancer. Fixing that leak would be more involved, and I'm honestly not to keen to spend time on this given that the Rubyserver is used less and less. Besides that, I didn't see any other Goroutines which still exist after the test. Furthermore, running the race detector on a loaded machine for about 10 minutes didn't surface any detected race at all. This may hint that #3637 (closed) doesn't exist anymore or that it was fixed by plugging the leaks. Hard to tell given that I wasn't able to reproduce the race without this MR in the first place.

In any case: part of #3637 (closed), but probably doesn't fix it.