[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/BUILD

@@ -45,6 +45,7 @@ cc_library(
     srcs = ["start_command.cc"],
     hdrs = ["start_command.h"],
     deps = [
+        ":background_service_client",
         ":base_command",
         ":local_assets_stream_manager_client",
         ":session_management_server",
@@ -78,6 +79,18 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "background_service_client",
+    srcs = ["background_service_client.cc"],
+    hdrs = ["background_service_client.h"],
+    deps = [
+        "//common:grpc_status",
+        "//common:status_macros",
+        "//proto:background_service_grpc_proto",
+        "@com_google_absl//absl/status",
+    ],
+)
+
 cc_library(
     name = "asset_stream_server",
     srcs = [
@@ -112,6 +125,8 @@ cc_library(
     deps = [
         ":base_command",
         ":multi_session",
+        ":session_management_server",
+        "//absl_helper:jedec_size_flag",
         "//common:log",
         "//common:path",
         "//common:status_macros",