@Override
    public void run() {
        Bucket toDelete = updateBuffer();
        createSnapshot(toDelete);
    }
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