- Added new parameter :paramref:`.mapper.confirm_deleted_rows`. Defaults

- Added new parameter :paramref:`.mapper.confirm_deleted_rows`.  Defaults
to True, indicates that a series of DELETE statements should confirm
that the cursor rowcount matches the number of primary keys that should
have matched;  this behavior had been taken off in most cases
(except when version_id is used) to support the unusual edge case of
self-referential ON DELETE CASCADE; to accomodate this, the message
is now just a warning, not an exception, and the flag can be used
to indicate a mapping that expects self-refererntial cascaded
deletes of this nature.  See also :ticket:`2403` for background on the
original change. re: #2403 fix #3007