[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/local_assets_stream_manager_client.h

@@ -20,7 +20,6 @@
 #include <memory>
 
 #include "absl/status/status.h"
-#include "grpcpp/channel.h"
 #include "proto/local_assets_stream_manager.grpc.pb.h"
 
 namespace grpc_impl {
@@ -32,8 +31,6 @@ namespace cdc_ft {
 // gRpc client for starting/stopping asset streaming sessions.
 class LocalAssetsStreamManagerClient {
  public:
-  explicit LocalAssetsStreamManagerClient(uint16_t service_port);
-
   // |channel| is a grpc channel to use.
   explicit LocalAssetsStreamManagerClient(
       std::shared_ptr<grpc::Channel> channel);