[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.

proto/background_service.proto

@@ -23,22 +23,16 @@ import "google/protobuf/empty.proto";
 service BackgroundService {
   // Exit is used to ask the service to exit. In the case of the process
   // manager, this cascades to all background processes.
-  rpc Exit(ExitRequest) returns (ExitResponse) {}
+  rpc Exit(google.protobuf.Empty) returns (google.protobuf.Empty) {}
 
   // GetPid is used to get the PID of the service process.
-  rpc GetPid(GetPidRequest) returns (GetPidResponse) {}
+  rpc GetPid(google.protobuf.Empty) returns (GetPidResponse) {}
 
   // HealthCheck is used to verify that the service is running. It returns an
   // empty protobuf if the service is ready to serve requests.
   rpc HealthCheck(google.protobuf.Empty) returns (google.protobuf.Empty) {}
 }
 
-message ExitRequest {}
-
-message ExitResponse {}
-
-message GetPidRequest {}
-
 message GetPidResponse {
   int32 pid = 1;
 }