allow deleted rows check to take place based on flag

[Mike Bayer]

see #2403 for original rationale to remote the "check deleted rows", we'll now do this:

    mapper(Parent, parent, confirm_deleted_rows=False)

the flag defaults to True and just emits a warning when the rows matched don't match.

[Mike Bayer]

• 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 2403 for background on the original change. re: #2403 fix #3007

→ <<cset d6618e411955>>