[cdc_stream] Automatically start service (#28)

Starts the streaming service if it's not up and running. This required
adding the ability to run a detached process. By default, all child
processes are killed when the parent process exits. Since detached
child processes don't run with a console, they need to create sub-
processes with CREATE_NO_WINDOW since otherwise a new console pops up,
e.g. for every ssh command.

Polls for 20 seconds while the service starts up. For this purpose,
a BackgroundServiceClient is added. This will be reused in a future CL
by a new stop-service command to exit the service.

Also adds --service-port as additional argument to start-service.

cdc_stream/session_management_server.h

@@ -36,7 +36,7 @@ class ProcessFactory;
 // - Background
 class SessionManagementServer {
  public:
-  static constexpr int kDefaultServicePort = 44432;
+  static constexpr uint16_t kDefaultServicePort = 44432;
 
   SessionManagementServer(grpc::Service* session_service,
                           grpc::Service* background_service,