Hopefully I'm not picking at a scab here.  I updated the dbm.sqlite
module in the sandbox.  It now orders by rowid instead of by key.
(I saw no performance penalty for the small table sizes I was using
to ordering.  I switched from ordering by key to ordering by rowid
based on Gerhard's comment.

I got a big performance boost on writes by only committing once every
100 calls to __setitem__.  I still commit when deleting keys and 
explicitly commit when closing.

The main performance bottleneck now appears to be keys() and iterkeys().
I don't see how to make them any simpler.  Oddly enough, it seems
that iterkeys() is slower than keys().  Maybe it's just lack of sleep
but I can't see why this is so.