Graceful shutdown service

gRPC Service on a shutdown should allow to complete all pending requests and close all used resources like DB (see https://github.com/opiproject/opi-spdk-bridge/issues/388) etc.
To gracefully stop gRPC server see [GracefulStop call](https://github.com/grpc/grpc-go/blob/v1.56.1/server.go#L1841)