Validator/authority contract (#3937)

* dir

* simple validator list

* stub validator contract

* make the engine hold Weak<Client> instead of IoChannel

* validator set factory

* register weak client with ValidatorContract

* check chain security

* add address array to generator

* register provider contract

* update validator set on notify

* add validator contract spec

* simple list test

* split update and contract test

* contract change

* use client in tendermint

* fix deadlock

* step duration in params

* adapt tendermint tests

* add storage fields to test spec

* constructor spec

* execute under wrong address

* create under correct address

* revert

* validator contract constructor

* move genesis block lookup

* add removal ability to contract

* validator contract adding validators

* fix basic authority

* validator changing test

* more docs

* update sync tests

* remove env_logger

* another env_logger

* cameltoe

* hold EngineClient instead of Client

* add a comment about lock scope