Alternative to Mysql that handles increment/decrement with new or existing keys
I'm not sure I fully understand the use-case, but in MySQL you can use:

    INSERT INTO balances (address, balance) VALUES (12, 15)
    ON DUPLICATE KEY 
    UPDATE balance = balance + 15;


... so you may not need a specialized DBMS for this sort of thing.

This type of statement is known as 'merge' (as in the SQL standard) or 'upsert'. See this Wikipedia article for more info, including details about the implementation in other relational database systems.