Allow to set createNewVersion (e.g. from save_callback)

My `save_callback` stores the result in a new subtable. I want to be able to enable versioning (through `onversion_callback` and `onrestore_callback`), but then I need to be able to tell the DC_Table to create a new version because my field value has changed.

PS: can you include this in the next 3.1.x?