Don't run pending_to_persistent for non-new objects

Don't run pending_to_persistent for non-new objects

Fixed fairly simple but critical issue where the
:meth:`.SessionEvents.pending_to_persistent` event would be invoked for
objects not just when they move from pending to persistent, but when they
were also already persistent and just being updated, thus causing the event
to be invoked for all objects on every update.

Fixes: #4489
Change-Id: Ibe147020aa62f7d605cb1029b7f3b776f42e6b43