Add a diagnostic check for updates to the same key out of timestamp order

The documentation in docs/transactions.dox states:

The commits to a particular data item must be performed in timestamp order.

Again, this is only checked in diagnostic builds and if applications violate

this rule, data consistency can be violated.

I attach a simple python unit test that illustrates this is not being checked in a diagnostic build.

The code should be brought in line with the documentation.