Turn FlushError for identity already exists into a warning.

Turn FlushError for identity already exists into a warning.

The condition where a pending object being flushed with an identity that
already exists in the identity map has been adjusted to emit a warning,
rather than throw a :class:`.FlushError`. The rationale is so that the
flush will proceed and raise a :class:`.IntegrityError` instead, in the
same way as if the existing object were not present in the identity map
already.   This helps with schemes that are uinsg the
:class:`.IntegrityError` as a means of catching whether or not a row
already exists in the table.

Fixes: #4662
Change-Id: I9314550b7b03d7f376ef35518da7542e0f2f7cb6