GRPC APIs to enable light gateways to verify state channels

[andymck]

3 APIs will be provided as follows:

1. is_valid: enables a light gateway to submit a state channel it received from a router to a validator and have the validator sanitise the SC and confirm it is valid

2. close: enables a light gateway to submit a state channel close txn to a validator and have the validator submit it to the chain

3. follow: enables a light gateway to submit a state channel ID of a state channel it received from a router and to receive streamed msgs pertaining to close related events for the specified state channel. Events include details of when the SC becomes closable, closing, closed and in dispute.