Improvement to min_required_balance: Make minimum balance for all savings accounts global

Currently when you have created savings account for a product, the minimum balance for that product is one capture and entered in min_required_balance for this savings account. Now if you change the minimum balance for this product, accounts already created are not affected by this changes i.e min_required_balance is not updated for the accounts in existence.

Any changes in min required balance should affect all accounts of that product i.e those accounts already created and those yet to be created. This will make sense and will be fair to all clients