Add liveness probe support to chaincode

[jt-nti]

Based on discussions with @y12studio in #98, it would be useful to support liveness probes for chaincode containers.

I don't think there is a simple way to implement this with current Fabric capabilities since the peer<->chaincode communication is via a long-lived grpc stream, and I'm not sure what chaincode language neutral alternatives might be possible instead.

One possibility is to update the Fabric protos to add a liveness service for chaincode implementations to (optionally?) support this requirement. The peer uses a separate operations service for health checks but that doesn't seem like a good approach for chaincode.