Eliminate all Instances of `Weight::zero()` (!197)

* refactor functions naming for pallet-membership

* refactor functions naming for pallet-certification

* fix tests with runtime-benchmark feature

* fix compilation with runtime-benchmarks feature

* update documentation

* remove handlers weights for pallet_membership

* remove handlers weights for pallet_identity

* remove handlers weights for pallet_certification

* add best practices for benchmarking

* add hooks benchmark pallet_universal_dividend

* add missing benchmarks pallet_universal_dividend

* update weights

* fix pallet_provide_randomness on_initialize weight

* fix pallet_identity weight::zero

* fix pallet_membership weight::zero

* fix pallet_certification weight::zero

* fix pallet_authority_members weight::zero

* fix pallet_identity weight::zero

* fix pallet_membership weight::zero