Graceful shutdown

[uatuko]

To ensure data consistency we need to intercept terminations signals and finish serving any in-flight requests and close connections to the DB before shutting down. This is very likely to require changes to gRPCxx.

Requirements

1. Intercept termination signals (SIGTERM, SIGINT, SIGQUIT) and;
   1. Stop accepting any further incoming requests after the termination signal
   2. Finish serving any in-flight requests
   3. Close DB connection
   4. Shutdown gracefully

[uatuko]

uatuko/grpcxx#42 should introduce a way to signal the grpc socket to shutdown using a std::stop_token.