Figure out / upgrade Berkeley DB

Our repository at http://svn.collab.net/repos/svn has been seeing some
nasty lossage ("unable to complete cache flush" or something -- Mike
has more details, can add them to this issue).  This repository uses
Berkeley DB 4.0.14.

In the meantime, we've had various reports from the field about
corruption in the db/changes table using BDB 4.1.24 and 4.1.25. 
People at Sleepycat said that what we saw was explainable if someone
had been accessing the repository while running recovery.  Not all
reporters think they were doing so, however.

So the situation right now is:

   1) Unexplained lockups with 4.0.14 on svn.collab.net.  

   2) Unexplained lockups of a different sort with 4.1.24/25, but it's
       not clear whether these are due to pilot error or not.

I'm not sure we want to spend time debugging 4.0.14, when it's such an
old release now.  It would be better to just require 4.1.x, if we can
establish that this is safe (when taken according to doctor's
instructions!).  This issue is for tracking the overall task of
digging ourselves out of BDB lossitude; it may get a new summary if we
agree that upgrading is the right course.