KafkaConsumer: store offsets when .finishing

Motivation:

This PR addresses issue #119.

The `KafkaConsumer` should not `fatalError` when storing offsets in
state `.finishing`.

See #119 (comment) for more information why.

Sources/Kafka/KafkaConsumer.swift

@@ -585,7 +585,9 @@ extension KafkaConsumer {
                 fatalError("Cannot store offset when consumption has been stopped")
             case .consuming(let client, _):
                 return .storeOffset(client: client)
-            case .finishing, .finished:
+            case .finishing(let client):
+                return .storeOffset(client: client)
+            case .finished:
                 fatalError("\(#function) invoked while still in state \(self.state)")
             }
         }