[contracts, client, node, apps] Move initial state validation to init() fn in appDefinition

[ArjunBhuptani]

Right now, we have to create new initial state validation libraries for every new application that we write. This makes it harder to add new apps and makes the node code less easy to understand. We also have a weird pattern where a proposal can be countersigned by a responder, even if they disagree with the proposed values.

We can move these validators into an init() function within the appDefinition which is call from within the propose protocol. This way, any app that makes it through the propose protocol, must necessarily have already been validated.

[LayneHaber]

Related issue for moving validation from listeners into validation: #1158

[ArjunBhuptani]

Deprioritizing this until we have a need to support many more apps