Implements cdc_stream stop-service. Also fixes an issue in the
BackgroundService implementation where Exit() would deadlock since
server shutdown waits for all RPCs to exit.
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.
