Cancel the impossible transitions along with transition approvals

[javrasya]

Description: When there more than one next state from the current state of the workflow and one of the transition approval is approved, django-river simply cancels all other peers and their future as well since they are impossible to reach from that moment on. It seems even though the transition approvals are canceled, their transitions still remain PENDING

Acceptance Criteria:

• Cancel all impossible transitions as well.

[JohnieBraaf]

This issue will be resolved by the commits referenced in this issue: https://github.com/javrasya/django-river/issues/188