HollowAPI will provide stale data when missing delta files

[yonatang]

When a refresh is triggered for an existing consumer and existing HollowAPI instance, but no delta file is available, the HollowAPI instance will keep producing stale data from the previous version.

If a delta file is available, or if a new instance of HollowAPI will be fetched by consumer.getApi(...), fresh data from the new version will be produced by the HollowAPI instance.

This behavior is consistent across many Hollow versions - I've checked it starting v4.0.0 and up to v6.1.0.

Please see a fully functional reproduction of the issue here: https://github.com/yonatang/hollow-delta-bug/tree/snapshot-load-issue