Emit after_rollback() event before snapshot removal

Emit after_rollback() event before snapshot removal

The state of the :class:`.Session` is now present when the
:meth:`.SessionEvents.after_rollback` event is emitted, that is,  the
attribute state of objects prior to their being expired.   This is now
consistent with the  behavior of the
:meth:`.SessionEvents.after_commit` event which  also emits before the
attribute state of objects is expired.

Change-Id: I9c572656ec5a9bfaeab817e9c95107c75aca1b51
Fixes: #3934