Report intermediate status of job

[felbalart]

While running a task waiting to be notified on completion, we should be able to send information to frontend about the current status or progress of the task.

This will allow, for example when processing a spreadsheet with 1000 rows, to show a progress bar or message like "Processing Row 550 of 1000..."

An approach for doing this could be recycle the same 'result' column, that is used for the job finish result. So in backend the job updates this value while running, and frontend receives it in every polling request.