Previous implementation only considered workflows that are chained to the
portal type:
- this is slower
- modification_date of old documents may change or disappear, preventing their
  reindexation
- edit_workflow was always considered, regardless it is chained or not

This commit makes getModificationDate more consistent: it simply iterates over
all workflow histories stored on the object.

When there is no workflow history, ZODB date is returned, which makes
getModificationDate useful for objects without any workflow chained, like
simulation movements.

Another optimization is that last history entry is accessed without loading
the whole history.