- Items that are too big for the cache are now no longer immediately
evicted; everything else is. This is probably not desirable, but it is
not unexpected.
- Multithreading now should work as expected; DbRefs are locked before
they are updated from the disk, and they are not evicted from the cache
if they are locked by another thread.
- The cache may be no smaller than 1024 bytes. Previously the caller of
DbOpen() could choose to disable the cache, and provisions were made in
the code to support this, but this is now no longer possible because
without the cache, there would be no way to know what files were open,
which could lead to a race condition if two threads open the same file.
