BerkeleyDB versions

OK, so 4.6.18 changes the lock management system in ways that are incompatible with previous releases. The advice from Oracle is to just use a read-only transaction (which is actually what we used to do, several revisions back). Of course with BDB 4.2 that causes the problem of log files staying open unless you apply the patch I wrote for that issue. It seems we may need to either resurrect that patch (it should still be in CVS) or drop support for DB 4.2 so we can rewrite the lock management (yet again) to support DB 4.6.

Our early tests with DB 4.6.1 thru 4.6.3 showed impressive performance gains, making it seem worthwhile. But we have no way of knowing how the current 4.6.18 will perform at the moment.