Increase rotation period (#2688)

* Cancel treasury burn and increase rotation period 

What this PR changes:

- Kusama treasury burn to be transferred to the Saociety pot: cancelled temporarily
- Increase the rotation period for candidates to show PoI FROM 3,3 days to 7 days.

* Update runtime/kusama/src/lib.rs

* Update lib.rs

Co-authored-by: Gavin Wood <gavin@parity.io>