Standardize cancellation semantics

[lblackstone]

Each provider includes a Cancel() method that signals for the provider to stop ongoing work, save current state, and shut down. This method is not implemented in some providers, and is handled inconsistently where it is implemented.

Each provider should create a "session" context that shares a lifecycle with the provider, and then join this context to request-specific contexts that are included with each gRPC method. This will allow downstream code to have a consistent cancellation signal that can be used to terminate gracefully.