Refactor ParkingLot so that it is flexible enough to support different underlying data-structures to store the waiter-list. We should likely do this by taking the WaiterList as some sort of template parameter or introducing a separate abstraction for it. 

Ensure that the existing implementation of the WaiterList (a std::list) can be used without any performance regressions, and demonstrate (but do not commit )that using a moodycamel::concurrentqueue or similar as the backing data structure can be done. 

Removing the dependency of Notifyable on the data-structure will also need to be done, or made generic, as part of this work.