Expire memoizations on setattr/delattr, check in delattr

Expire memoizations on setattr/delattr, check in delattr

Fixed bug where declarative would not update the state of the
:class:`.Mapper` as far as what attributes were present, when additional
attributes were added or removed after the mapper attribute collections had
already been called and memoized.  Addtionally, a ``NotImplementedError``
is now raised if a fully mapped attribute (e.g. column, relationship, etc.)
is deleted from a class that is currently mapped, since the mapper will not
function correctly if the attribute has been removed.

Change-Id: Idaca8e0237b31aa1d6564d94c3a179d7dc6b5df9
Fixes: #4133