Implements gRPC Health check using standard grpc library. Reuses the
health as reported by gosundheit health checker. Grpc health server
implements also streaming Watch method, so it was necessary to add
Stop() failback to stop server even if some clients ignore NOT_SERVING
status. Timeout for graceful shutdown was chosen to match timeouts for
http and telemetry servers.
Signed-off-by: Robert Moucha <robert.moucha@gooddata.com>