pow: support uniform tie breaking in fork choice (#7073)

* pow: support uniform tie breaking in fork choice

* Update client/consensus/pow/src/lib.rs

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

* Refactor fetch seal

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