de-memoize _proxy_key when new annotations are added

de-memoize _proxy_key when new annotations are added

Fixed regression from 1.3 where the column key used for a hybrid property
might be populated with that of the underlying column that it returns, for
a property that returns an ORM mapped column directly, rather than the key
used by the hybrid property itself.

Fixes: #11728
Change-Id: Ifb298e46a20f90f6b6a717674f142a87cbceb468
(cherry picked from commit ffc7e8d73b30ea45fb03e0727b9fe96b6b8d4cfa)
(cherry picked from commit 5b1758a9bb8952adca91a95483ec1d11a66ad1e2)