incrementally load steps

[jtoy]

Right now for the web ui, when you pass a simulation id, it loads all steps from the backend service which can be redis/api end point.
Instead use a variable called steps_per_call and set to 50. It loads chunks of steps_per_call at a time, so as to give a smaller chunk of data from the server at a time. This will allow the service to load much faster. This should work on local redis mode and in api mode.
current loader is web/src/components/RenderLevel.tsx