Replace RollingSessionWindow in approval-voting with RuntimeInfo (#…

…7123)

* Replace `RollingSessionWindow` with `RuntimeInfo` - initial commit

* Fix tests in import

* Fix the rest of the tests

* Remove dead code

* Fix todos

* Simplify session caching

* Comments for `SessionInfoProvider`

* Separate `SessionInfoProvider` from `State`

* `cache_session_info_for_head` becomes freestanding function

* Remove unneeded `mut` usage

* fn session_info -> fn get_session_info() to avoid name clashes. The function also tries to initialize `SessionInfoProvider`

* Fix SessionInfo retrieval

* Code cleanup

* Don't wrap `SessionInfoProvider` in an `Option`

* Remove `earliest_session()`

* Remove pre-caching -> wip

* Fix some tests and code cleanup

* Fix all tests

* Fixes in tests

* Fix comments, variable names and small style changes

* Fix a warning

* impl From<SessionWindowSize> for NonZeroUsize

* Fix logging for `get_session_info` - remove redundant logs and decrease log level to DEBUG

* Code review feedback