Implementation for a simple order-preserving encryption scheme.

This is meant for use when encrypting the current time within the
period in order to get a monotonically increasing revision counter
without actually revealing our view of the time.

This scheme is far from the most state-of-the-art: don't use it for
anything else without careful analysis by somebody much smarter than
I am.

See ticket #25552 for some rationale for this logic.