Preserve the observer associated with an existing topic

[onlybakam]

when a new request is executed, we need to make sure the observer intially associated with a topic is preserved. currently, on a new request, we disconnect all the clients and then reconnect them and associate them with the latest observer. When a subscription message is received, we then fetchthe observer with:

this.topicObserver.get(topic);

and call

observer.next(parsedMessage);

which will trigger a specific handler configured for that observer (the handler that subscribed to the observer returned by the execute function in QueryManager.startGraphQLSubscription.

The change keeps track of the previous value of topicObserver and retains the observer associated with an existing topic.

[manueliglesias]

👍