Backporting better BTree conflict resolution tests from

Zope3/ZODB4.  Many test fixes from Jeremy (in particular,
most of the tests weren't getting run), and new tests from
Tim.  Note that the new testEmptyBucketConflict() actually
shows that a particular rare case of conflict resolution
doesn't work correctly now.  A fix for that comes next,
along with reversing the sense of this new test.