issue270_incremental_diff

It is easier to get nearest snapshot to create a snapshot for the specific date.
Snapshot for date D created as (pseudo-code):
```
base_snaphot = nearest_snaphot_for_the_date(D5)        
result_snapshot = base_snapsot + added_diff_files_between(base_snapshot.date, D)  
                                - removed_diff_files_between(base_snapshot.date, D)
```