Use Vote Ancestries Directly in Weight Calculations (#862)

* Use more accurate weight calculation in declared weight

* Remove session length and validator set size config constants

* Remove config params from mock

* Allow specifying total number of votes-ancestries per justification

* Change limits used during benchmarking

* Regenerate weights

* Use simplified weight annotation

* Remove comment

* Address leftover TODO

* Prevent possible divide by zero errors

* Use correct argument order in weight declaration