The checkin implies:

  - the "DBTab" package has been removed.  It is no longer necessary.
    Instead, we use ZODB 3.6's built-in multidatabase support.

  - ZODBMountPoint no longer monkey-patches ZODB to allow for mount
    support.

  - We temporarily hack setup.py to allow ZODB 3.6.0b1 to be compiled
    (it expects to be compiled via ZPackage).