batch signalling channel messages and send them at most every second

Please don't review this just yet...the code isn't tidy or tested. Just for experimentation.
At some point we should also check that parsing succeeded and that the resulting JS object is well-formed (i.e., that it has the properties we expect and that those properties' values have the correct types).  I think it's fine to leave all that as a TODO for now, but before long it would be worthwhile to validate this over-the-wire input before consuming it.