- significantly rework the approach to collection events and history within DynamicAt...

- significantly rework the approach to collection events and history within DynamicAttributeImpl
- Fixes to the "dynamic" loader on :func:`.relationship`, includes
that backrefs will work properly even when autoflush is disabled,
history events are more accurate in scenarios where multiple add/remove
of the same object occurs, as can often be the case in conjunction
with the association proxy.  [ticket:2637]