Significant changes in ETL algorithm:
1. Added _meta collection for each collection. Stores aggregated data as needed. For now it's max_date for contracts by region.
2. Because of (1) file listing and filtering is now very fast (no sorting needed).
3. Added inc_files() and all_files() functions that form _meta collection and list files as needed.
4. _meta collection now updated after each file load.
5. Removed documents list, now load() is called for each document right after it is transformed.
6. File list is now list of tuples (filename, region). This is needed for loading and _meta updating.