session-info: add new fields + migration (#4545)

* session_info: v2 + migration

* use primitives::v2

* use polkadot_primitives::v2

* impl primitives::v2

* fix approval-voting tests

* fix other tests

* hook storage migration up

* backwards compat (1)

* backwards compat (2)

* fmt

* fix tests

* FMT

* do not reexport v1 in v2

* fmt

* set storage version to 1

Co-authored-by: Javier Viola <javier@parity.io>