Consider batching create/update record requests in JS

[snickell]

Right now we do a browser=>rails=>db round-trip for every create_record call, which there could be very many per second. If we don't like the load we're seeing on the server, since we don't have onRecordEvent anyway, it might make sense to cache create_record calls, say every 0.5s, and batch execute them (which our backend API allows for).

We can decide this as we investigate switching a small number of projects on prod, and see how the loading looks.