babe: only process vrf on module finalization (#11113)

* babe: only process vrf on block execution finalization

* babe: rename CurrentBlockRandomness to PreviousBlockRandomness

* babe: add test for initialization ordering

* babe: rename PreviousBlockRandomness to ParentBlockRandomness

* babe: re-add CurrentBlockRandomness with deprecation notice

* babe: export CurrentBlockRandomness

* babe: silence deprecation warning when exporting CurrentBlockRandomness

* babe: suggestion from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* babe: flatten nested option

* babe: rustfmt

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>