As part of PM-1645 we are trying to move towards unifying all cluster-wide cached information to be stamped with configOpTime (or clusterTime). This has various benefits, but the most important of them is the ability to infer causality due to the ordering of opTimes.

In this model, the "cache generation" in the authorisation manager would be easier to think of as the opTime of when the last change to the auth data happened. This change makes the UserCacheInvalidatorJob expect both OID and Timestamp so that in later 4.4 releases we can change it to be a Timestamp without backwards compatibility implications.