Story: Extend Delta feature to be able to work with a copy of live store

Nobody has claimed this yet.

Right now Delta can work effectively only against live/online store. Some of our clients are concerned that the tool installs additional tables and triggers on their stores M1. Thus the safest way is to do main migration from a server's copy (staging server) of the store. This, in turn, makes impossible for Delta to catch new records that came to live store during Data migration.

*The solution* is described in the next steps.

1. Merchant sends his current DB dump of M1 store to a developer
2. The developer migrates the first DB dump to M2 store
3. The developer makes customization asked by merchant to M2 store
4. Since item 3 takes some time to implement, for this period new records (customers, orders, etc.) most likely appeared on live M1 store
5. Merchant sends his second DB dump of M1 store to a developer
6. The developer by the Data Migration Tool compare the first and the second DB copies of M1 to find the new records (customers, orders, etc.)
7. The new Delta records are migrated to Magento 2 DB

In the #316 users created a custom script with a similar approach
:warning: Step number *6* can take a lot of time with big DB. This can be a *major drawback* of the whole feature

No files or tests are named. Start by reading issue #316 and tracing the existing Delta migration flow; completion means comparing two M1 database copies and migrating only records newly present in the second copy to M2, while addressing the stated large-database concern.

Written by the indexing model from the issue text.