- attributes now has an "active_history" flag. This flag indicates that when new value is set or the existing value is deleted, we absolutely need the previous value to be present, including if it requires hitting a lazy loader. Since somewhere around 0.4 we had not been loading the previous value as a performance optimization.

- the flag is set by a ColumnLoader which contains a primary key column.  This allows the mapper to have an accurate record of a primary key column when _save_obj() performs an UPDATE.
- the definition of who gets "active_history" may be expanded to include ForeignKey and any columns participating in a primaryjoin/seconddary join, so that lazyloaders can execute correctly on an expired object with pending changes to those attributes.
- expire-on-commit is why this is becoming a more important issue as of late
- fixes [ticket:1151], but unit tests, CHANGES note is pending

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