Else those non-current entries can be used to serve a loadBefore request
with data, while, after pack that loadBefore request must return "data
deleted" if requested object has current revision >= packtime.

Fixes checkPackVSConnectionGet from ZODB from https://github.com/zopefoundation/ZODB/pull/322
which, without this patch fails as e.g.

    Failure in test checkPackVSConnectionGet (ZEO.tests.testZEO.MappingStorageTests)
    Traceback (most recent call last):
      File "/usr/lib/python2.7/unittest/case.py", line 329, in run
        testMethod()
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/tests/PackableStorage.py", line 636, in checkPackVSConnectionGet
        raises(ReadConflictError, conn1.get, oid)
      File "/usr/lib/python2.7/unittest/case.py", line 473, in assertRaises
        callableObj(*args, **kwargs)
      File "/usr/lib/python2.7/unittest/case.py", line 116, in __exit__
        "{0} not raised".format(exc_name))
    AssertionError: ReadConflictError not raised