[beta] Kovan HF (#7234)

* Kovan HF.

* Bump version.

* Fix aura difficulty race (#7198)

* Fix Aura difficulty race

* fix test key

* extract out score calculation

* fix build

* Update kovan HF block number.

* Add missing byzantium builtins.

* Bump installers versions.

* Increase allowed time drift to 10s. (#7238)