to be reused if the DB is dropped and recreated, since when that
happens, auto_increment history is lost and it will go back to using
the latest highest index in the table. Usually not a problem, but
since we cross index three other tables using the experiment index,
this causes quite a bit of grief.

So, my solution is to do my own auto_increment using the
experiment_stats table (locked of course), which we never delete
entries from without deleting all entries from the other cross
referenced tables.

    DBQueryFatal("select MAX(exptidx) from experiment_stats");

I also added a sanity check to make sure the new index is not
currently in use in any of the tables. I also cleaned up the
error path when something goes wrong.