Spotted by rami.ojares@gmail.com

A persistent multi-version map: avoid 'magic' casting in the comparator, because strictly Collections.binarySearch could swap the parameter objects in the call to the compare method. This should also simplify porting the code to C.

(a) any accidental further access to it
(b) throwing an exception if the client code calls close() more than once

Fix for a bug where we would sometimes use the wrong unique constraint to validate foreign key constraints.