Shift Session Manager Pallet

THIS PALLET IS NOT INTENDED TO BE USED IN PRODUCTION

The pallet does not provide any calls or runtime storage entries. It only provides implementation of the pallet_session::SessionManager. This implementation, starting from session 3 selects two thirds of initial validators and changes the set on every session. We are using it at our testnets (Rialto and Millau) to be sure that the set changes every session. On well-known production chains (like Kusama and Polkadot) the alternative is the set of nPoS pallets, which selects validators, based on their nominations.