Add merge functionality to snapshot

[toefel18]

Provide a method on Snapshot to create a new snapshot that is a merge of itself and another snapshot:

public class Snapshot {
    public Snapshot merge(Snapshot other) {
             return newSnapshotInstanceWithMergedData;
     }
}

Users can then track the statistics since program start by calling getSnapshotAndReset() periodically and merge it with a global instance of Snapshot.