ReactRelayRefetchContainer refetch callback disposed

I set up loading overlay before `refetch` call and remove it in `callback`:
```js
this.setState({ loading: true });
this.props.relay.refetch(null, null, (error) => {
this.setState({ loading: false });
});
```
In some cases `callback` is never called although component receives new props.
I belive it happens when there are new data entries after fetch.
During [this check](https://github.com/facebook/relay/blob/master/packages/react-relay/modern/ReactRelayRefetchContainer.js#L174):
```js
static getDerivedStateFromProps(
...
if (
prevState.relayEnvironment !== relay.environment ||
prevState.relayVariables !== relay.variables ||
!areEqual(prevIDs, nextIDs)
) { ... }
}
```
component gets new resolver and [disposes callback here](https://github.com/facebook/relay/blob/master/packages/react-relay/modern/ReactRelayRefetchContainer.js#L135):

```js
componentDidUpdate(prevProps: ContainerProps, prevState: ContainerState) {
if (this.state.resolver !== prevState.resolver) {
prevState.resolver.dispose();
this._queryFetcher && this._queryFetcher.dispose();
this._refetchSubscription && this._refetchSubscription.unsubscribe();
this._subscribeToNewResolver();
}
}
```
So, there is no guarantee that callback would be called after each refetch?
I use React Modern with Found-Relay

Start in packages/react-relay/modern/ReactRelayRefetchContainer.js, especially getDerivedStateFromProps and componentDidUpdate. Reproduce a refetch that receives new data entries, then trace disposal of the resolver, query fetcher, and refetch subscription. Done means the callback behavior is defined and covered for this case without leaving the loading overlay active.

Written by the indexing model from the issue text.