pow: replace the thread-base mining loop with a future-based mining worker (#7060)

* New worker design

* Remove unused thread import

* Add back missing inherent data provider registration

* Add function to get a Cloned metadata

* Add some docs

* Derive Eq and PartialEq for MiningMetadata

* Fix cargo lock

* Fix line width

* Add docs and fix issues in UntilImportedOrTimeout

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

Co-authored-by: David <[email protected]>

* Add back comments

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: David <[email protected]>