1. The saving was performed in (tpc)_finish.  It is important that this
   method do as little as possible because it cannot fail!

2. The algorithm for deciding how often to save was broken. For large
   databases, for which saving periodically is desireable, the period
   was set so high that the index was effectively never saved.

It might be nice to save periodically, but doing so is tricky, since
we really don't want to do it during commit.  Until we figure out how
to do this right, it is better not to try.

In the mean time, we save on close and on pack, which is proably often
enough in most cases.