Instance mode support

[rmichela]

gRPC binds to one instance of a service, effectively making each service a singleton. While good for performance, this can cause problems when less experienced developers accidentally leak state between service operations.

Add support for the following instance modes

• Singleton - the default
• Per-call - create a new service instance for every RPC
• Per-session - (hardest) create a new service instance for each distinct client session

[rmichela]

Implemented in #65