control over write volatility

I don't know the proper term for what I called write volatility, so let me explain.

If someone repeatedly calls db.set() with the same key, dirty has two options: 

1) to write every set() to disk
2) to write only the last set() to disk

In certain situations both (1) and (2) can be useful, so I'd like to have an option when I open a database.
