Merge cdc_stream into asset_stream_manager (#26)

Adds start and stop commands to asset_stream_manager.
asset_stream_manager will be renamed to cdc_stream next.

asset_stream_manager/session_management_server.h

@@ -36,6 +36,8 @@ class ProcessFactory;
 // - Background
 class SessionManagementServer {
  public:
+  static constexpr int kDefaultServicePort = 44432;
+
   SessionManagementServer(grpc::Service* session_service,
                           grpc::Service* background_service,
                           SessionManager* session_manager);