[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.

common/remote_util.cc

@@ -75,6 +75,7 @@ absl::Status RemoteUtil::Scp(std::vector<std::string> source_filepaths,
 
   // -p preserves timestamps. This enables timestamp-based up-to-date checks.
   ProcessStartInfo start_info;
+  start_info.flags = ProcessFlags::kNoWindow;
   start_info.command = absl::StrFormat(
       "%s "
       "%s %s -p -T "
@@ -147,6 +148,7 @@ ProcessStartInfo RemoteUtil::BuildProcessStartInfoForSshInternal(
       QuoteForWindows(ssh_command_), quiet_ || verbosity_ < 2 ? "-q" : "",
       forward_arg, QuoteForWindows(user_host_), ssh_port_, remote_command_arg);
   start_info.forward_output_to_log = forward_output_to_log_;
+  start_info.flags = ProcessFlags::kNoWindow;
   return start_info;
 }