Avoid the monkey patch by creating a subclass of Connection and
installing it as the klass attribute of ZODB's DB class.  I think this
works correctly in conjunction with Zope configuration (the tests
pass) -- but I'm not qualified to say for sure.

Rework the logic of MountConnection as part of the restructuring.  The
_root_connection attribute always points to the root, never to None;
so the root object points to itself.  This change necessitated a
chance in DBTab.ClassFactories.  Add some comments there to explain
what's going on.

XXX The close() method on MountConnection is re-implementing two
methods in modules.  If those implementations change, it would need to
change, too.  We ought to find a better way to integrate it.

XXX Perhaps the mounted connection stuff should be folded into ZODB
proper.