private void createSnapshot(Bucket toDelete) {
        Bucket toPresent = new Bucket();
        for (Bucket b : buffer) {
            if (b == null) continue;
            toPresent.merge(b);
        }
        dimensions.updateDimensionPersistence(toDelete, toPresent);
        currentSnapshot.set(toPresent);
    }
Worker thread to collect the data stored in worker threads and build
snapshots for external consumption. Using the correct executor gives the
necessary guarantees for this being invoked from only a single thread.

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