void updateDimensionPersistence(Bucket toDelete, Bucket toPresent) {
        updatePersistentData(toDelete);
        padPresentation(toPresent);
    }
The persistence layer for metrics. Both CPU and memory hungry, but
it runs in its own little world.

@author Steinar Knutsen
container-core/src/main/java/com/yahoo/metrics/simple/DimensionCache.java