Adder collator improvements (#1896)

* Fixes bug that collator wasn't sending `Declare` message

* Set authority discovery config

* Fixes bug that collator wasn't sending `Declare` message

* Adds real overseer feature and makes the wasm_validation fail with a
proper error

* Adds README

* Remove debug stuff

* Add feature

* Make adder collator use the correct parent when building a new block