Fixes a number of issues related to staging and caching.
The most important fix is related to ensuring that we don't clear out the staged results for a given tag when one tagged cache in a staged group is null. Previously, if one cache had the same tag as another, if the first was null, it could also cause the system to wrongly conclude that the second was null as well.

In addition, there were some improvements that ensure that we are not needlessly generating new tag versions when we are only trying to read the version to see if it is valid.