Proper two-phase transactions.

[gaperton]

Looks like we finally need them for the scenario where three collections form the tree of models across them by model id. Trick is that we need to mark parents as 'changed', which triggers extra updates causing expensive renders.

So, the node starting the transaction must put all owners in transaction mode. And we need to close owner's transactions properly. And it must be done in the way that performance won't be hurt.