Use past switch blocks up to an emergency restart in the Era Supervisor

The Era Supervisor currently only collects switch blocks up to the last activation point when creating a new era. This is a remnant of when we weren't able to tell when the last emergency restart was and thus wouldn't be able to tell whether switch blocks before the last activation point were valid. Currently, we have API for that in the chainspec loader, so the Era Supervisor should also only stop at an emergency restart, not at regular upgrades when walking back the switch blocks (similar to what `get_era_validators` does).