feat: start adapting load_flow for rr-backend; not finished

* testing:
  * add a simple test for it in rr_test
  * add a timeout for the rr test for now with the `ntest` crate
* impl:
  * start refactoring flow_preloader; not finished

Author: alehander92

src/db-backend/Cargo.lock

@@ -51,6 +51,8 @@ vfs = "0.12"
 
 expanduser = { version = "1.2.2", optional = true}
 
+ntest = "0.9.3"
+
 [lib]
 name = "db_backend"
 path = "src/lib.rs"