Follow up to testground/sdk-go#31 and #1154

At the moment we have 2 methods, FetchAllEvents and SubscribeEvents, that consumers use to get events from the sync service via the go-sdk.

We can probably unify them into just one method which stops consuming events when it's context is canceled (i.e. SubscribeEvents), but that would also require a slight refactor on Testground side where we use FetchAllEvents.