Add graceful gRPC server example

[ting-hong-shieh]

Closes #5.

Summary

• Adds a standalone gRPC example module without changing tableflip's core dependencies or Go 1.14 requirement.
• Transfers the listening socket during SIGHUP upgrades and drains active RPCs with GracefulStop.
• Marks the standard gRPC health service as not serving before shutdown and force-stops RPCs after a configurable deadline.
• Includes server reflection, health-check and upgrade commands, and unit tests for graceful and timed-out shutdown paths.

Validation

• go test ./... in the gRPC example module
• go test -race ./... in the gRPC example module
• go vet ./... in the gRPC example module
• go build ./... in the gRPC example module
• go test ./... and go vet ./... in the tableflip root module
• End-to-end SIGHUP upgrade: PID changed from 15121 to 15157 and grpc.health.v1.Health/Check returned SERVING before and after the handoff
• git diff --check