[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/stop_command.cc

@@ -21,6 +21,9 @@
 #include "common/log.h"
 #include "common/path.h"
 #include "common/status_macros.h"
+#include "grpcpp/channel.h"
+#include "grpcpp/create_channel.h"
+#include "grpcpp/support/channel_arguments.h"
 #include "lyra/lyra.hpp"
 
 namespace cdc_ft {
@@ -58,7 +61,13 @@ void StopCommand::RegisterCommandLineFlags(lyra::command& cmd) {
 absl::Status StopCommand::Run() {
   LogLevel level = Log::VerbosityToLogLevel(verbosity_);
   ScopedLog scoped_log(std::make_unique<ConsoleLog>(level));
-  LocalAssetsStreamManagerClient client(service_port_);
+
+  std::string client_address = absl::StrFormat("localhost:%u", service_port_);
+  std::shared_ptr<grpc::Channel> channel = grpc::CreateCustomChannel(
+      client_address, grpc::InsecureChannelCredentials(),
+      grpc::ChannelArguments());
+
+  LocalAssetsStreamManagerClient client(channel);
 
   std::string user_host, mount_dir;
   RETURN_IF_ERROR(LocalAssetsStreamManagerClient::ParseUserHostDir(