Fix subscription event delivery when some subscriptions expire

[prog-supdex]

Fix #52

Before this fix, if a subscription expired after checking at the existing moment during processing, no events would be delivered to any subscribers

[prog-supdex]

GitHub workflow installs graphql 2.5 (here GraphQL-Ruby ~> 2.3), which brings stricter validation for subscriptions here and here

According to the validations, subscription operations must have only one root field
This means we can not use multiple subscription fields in a single query like:

subscription SomeSubscription {
  productCreated { id title }
  productUpdated { id }
}

I have updated the tests (stat_spec and anycable_spec) to follow this limitation