[node] Challenge protocol

[LayneHaber]

Helper methods for initiating and responding to challenges needed on the node package.

This will be important for reclaiming collateral from offline users, or breaking even in the case of linked transfers if the sender does not come back online.

Will likely need the following methods:

• getActiveChallenges -- returns all active challenges
• initiateChallenge(appId) -- begins a challenge for a specific app
• respondToChallenge(appId, appState) -- responds to challenges that have been initiated, may not need to provide a state here if it can be retrieved from the store