request queue leftovers

I'm not sure if this has any external effects (other than making our own testing harder) but there seem to be some cases where the requestQueue in the renderer redux state gets some stragglers. Noted by @byronz in #1001 when trying to wait for all callbacks to have completed prior to taking a snapshot, there are occasionally some requests with controllerId: null left over after everything has, in fact, completed.

These appear to be coming from this block:

https://github.com/plotly/dash/blob/046c438928c17e0f7faf5343f5f4e6d98d0c9efd/dash-renderer/src/actions/index.js#L926-L952

Apparently there are some implications 😏
My guess is that, because dispatch is not synchronous, updateOutput doesn't yet have the request present in order to clear it, so when it gets added later it just sticks around forever. I don't understand why we need this dummy request in the first place... to be investigated.

Start in dash-renderer/src/actions/index.js at lines 926-952 and trace how requestQueue entries are added and cleared around updateOutput. Reproduce the snapshot-wait scenario described in #1001, then verify that completed callbacks leave no requestQueue entries with controllerId: null.