core: Check if context is canceled while waiting for app.started

[albrow]

The fix implemented in #560 could break graceful shutdowns. We need to account for the fact that the context could be canceled before the app is actually started. So something like this:

select {
    case <-app.ctx.Done():
        return nil, context.Canceled
    case <-app.started:
        // continue as normal...
}

See also #161