Validator/authority contract

[keorn]

Ability to use a contract at a specified address to determine the current list of validators (renamed from authorities) as an alternative to fixed spec list. The test contract is here.

Another change is that Engine now holds a Weak<Client> rather than IoChannel.

Do not merge for 1.5, goes after #3932.<\s>

[coveralls]

Coverage increased (+0.9%) to 86.689% when pulling b7996fa on validator-lookup into bce69b2 on master.

[rphmeier]

Another change is that Engine now holds a Weak rather than IoChannel.

I really don't agree with this -- Engine is supposed to be a relatively agnostic consensus mechanism which the client and other related services can leverage. Any communication with other parts of the codebase should be done through a more general mechanism. I'm not sure IoChannel is quite right, but requiring a dependency on Client in the engine is overly restrictive.

[gavofyork]

so Engine is indeed meant to be a resource, not a coordinator.

however, Engine and Client are very naturally tightly coupled and this becomes especially evident when aspects of the consensus algorithm (Engine) depend on aspects of the present state (Client) rather than just the other way around, as with the Ethash system. there could be a much tighter trait made from Client (EngineClient?) which only exposed a specific subset that could be useful to Engine (and that might be worth considering for a separate refactoring PR), but right now i fear this is a best we can manage.

[coveralls]

Changes Unknown when pulling 38ea976 on validator-lookup into ** on master**.

[keorn]

Now the two Engines hold EngineClient.
Client is passed since when necessary (validator set is in the state) an Engine will add_notify which inspects the validators using a closure, similar to the updater.

[coveralls]

Coverage increased (+0.2%) to 86.737% when pulling fa59451 on validator-lookup into 4c94878 on master.

[gavofyork]

good to have a comment here to ensure any newcomer understands what the parens are for.

[gavofyork]

minor grumble. ltgm otherwise.