[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/start_command.h

@@ -20,6 +20,10 @@
 #include "absl/status/status.h"
 #include "cdc_stream/base_command.h"
 
+namespace grpc {
+class Channel;
+}
+
 namespace cdc_ft {
 
 // Handler for the start command. Sends an RPC call to the service to starts a
@@ -34,6 +38,9 @@ class StartCommand : public BaseCommand {
   absl::Status Run() override;
 
  private:
+  // Starts the asset streaming service.
+  absl::Status StartStreamingService();
+
   int verbosity_ = 0;
   uint16_t service_port_ = 0;
   uint16_t ssh_port_ = 0;