Support multiple inheritance.

[bckohan]

The current barrier is that the polymorphic_ctype_id fields from multiple polymorphic parents conflict.

A solution here might relate to #429

[JohananOppongAmoateng]

@bckohan I think this is issue is better slatedfor version 5

[bckohan]

@bckohan I think this is issue is better slatedfor version 5

Maybe! I think we can probably support this with no impact to existing models. If that turns out to be true then I think its fine to push into 4.6

[bckohan]

@JohananOppongAmoateng Having looked more into this - you're right. I think we could solve this in the current architecture but its really more trouble than its worth. If in version 5 we eliminate the need for polymorphic_ctype_id this issue will be solved naturally.

Punting to 5.0.