Finality votes cache (#116)

* removeInMemoryStorage + extract Kovan stuff to runtime

* removed comment from the future

* remove redundant conversions

* remove redundant `u8 as usize`

* remove redundant `u8 as usize`

* Update modules/ethereum/src/mock.rs

Co-authored-by: Hernando Castano <[email protected]>

* use hex-literal in kovan config

* cargo fmt --all

* extracted insert_header

* cargo fmt --all

* finality cache

* cargo fmt --all

* cargo fmt --all

* impl Default for FinalityVotes

Co-authored-by: Hernando Castano <[email protected]>
Co-authored-by: Hernando Castano <[email protected]>