Add sanity checking (validation) code to BTree Manager

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The BTree Manager is easily the most complex of the SimpleDBM modules. If
things go wrong it is very hard to pinpoint where the problem lies. 

Already several measures are present:

Trace messages have been added recently.
Node level validations are done if appropriate flag is set at run time.
Simple checks are performed when inserting new entries in a page - also
when a flag is set.
Number of assertions have been increasing.

But these measures are all ad-hoc. We need a more systematic approach so
that we get the maximum benefit at minimum overhead. For instance, it only
makes sense to validate stuff if things have changed.

Original issue reported on code.google.com by [email protected] on 2 Aug 2008 at 12:14