Look at the upgrade go-ahead and restriction signals (#517)

* Look at the upgrade go-ahead and restriction signals

* Update Cargo.toml

* Drop old docs for validation code

* Update tests

* Fix typo

* Add doc-comments for read_optional_entry

* Add a note about ValidationData

* Introduce migration for removing unused storage entry

* Fix indentation

* Use intra-doc link syntax

* Double-check that GoAhead signal is not spurious

* fmt

* Drop commented code

* Fix typos

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Add a weight for StorageVersion write

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

Co-authored-by: Chris Sosnin <chris125_@live.com>
Co-authored-by: Chris Sosnin <48099298+slumber@users.noreply.github.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>