Many database-interface improvements.

1 Added an explicit close to the Indexer class.  This was handled
   by garbage collection before.

 2  Added an MKBackendError exception that gets thrown on some metakit errors.
   Should there be a general rdbms exception?

 3 Added a sanity check when creating metakit tables in __getview
   There are some test cases that create columns of the same name
   with different metakit types.  This can cause crashing issues with
   older version of metakit.  We catch these before hand and raise
   an exception.

 4 metakit db's cannot be weakref'd so this was removed

 5 fixed a metakit.append, metakit must append lists, objects or
   dictionaries, it can't handle scalars.
   sv.append(int(entry))
    became
   sv.append((int(entry),))

 6 To make it easier to compare to the other backends
  Class.keyname is changed to Class.key

 7 Fixed Class.lookup, sometimes it would claim that a valid
  row was not valid (an _isdel row _property of 0 would
  be reported as 1) This is because metakit's view.find
  operation was returning bad results.  This should be
  view.select or view.find on a single property.

 8 calling create with no parameters raises a value error
  I'm not sure if this is appropriate, but it fixes
  a regression test :)

 9 The get method was only converting results for commited
   values.  uncommited values were not being converted
   using the metakit conversion table

 10 Added a check to the Class.__init__ to raise a ValueError
    if the database already has a class of the same name.

 11 Boolean and Number types can now have null values.  This
    is a backwards incompatible fix in that old databases
    won't work correctly.

    The fix is simple.  For a boolean column, 0 is now None
                                              1 is False (returns 0)
                                              2 is True (returns 1)
                        For a numeric column, 0 is now None
                                              values 0 get returned as value-1
                                              values < 0 get returned as value
   Set the BACKWARDS_COMPATIBLE flag to False to enable this fix.

 12 Enumerated READ and READWRITE for the getview and getindexedview
    These will probably be removed because they are not used
 Known Current Bugs:
       It is currently is not possible to retire an id with name X
       and add a new unretired property with name X.
       Confused?  Here is the regression test:

        self.assertRaises(ValueError, self.db.user.create)

        newid = self.db.user.create(username="spam")
        self.assertEqual(self.db.user.lookup('spam'), newid)
        self.db.commit()
        self.assertEqual(self.db.user.lookup('spam'), newid)
        self.db.user.retire(newid)
        self.assertRaises(KeyError, self.db.user.lookup, 'spam')

        # use the key again now that the old is retired (metakit FAILS!!)
        newid2 = self.db.user.create(username="spam")
        self.assertNotEqual(newid, newid2)
        # try to restore old node. this shouldn't succeed!
        self.assertRaises(KeyError, self.db.user.restore, newid)

        self.assertRaises(TypeError, self.db.issue.lookup, 'fubar')

     Boolean and number values will not return None (now fixed but
      breaks backwards compatibility)

     This causes some regression tests to fix, namely:

     def testPasswordUnset(self):
        x = password.Password('x')
        nid = self.db.user.create(username='foo', password=x)
        self.db.user.set(nid, assignable=None)
        self.assertEqual(self.db.user.get(nid, "assignable"), None)

Author: Brian Kelley