* op-node: refactor finality to encapsulate and simplify EngineQueue

* op-node: add lock to make concurrent use of Finalizer by plasma backend safe, and rename receiver-method var names

* op-node: reintroduce instant L2 finality check upon L1 signal, reintroduce extra check to handle contrived test

* op-node: fix plasma finalization test setup

* semgrep fix

* op-node: link TODO issue