Asynchronous imports

[nilshoerrmann]

As mentioned in #45, we are synchronising Symphony with a large product database containing a few thousand items. Running the importer will freeze the backend for quite some time so I thought about asynchronous imports:

• Running an importer will make a AJAX request to the server initialising the import.
• This background import will write the import stats (the count of imported entries and the count of all entries) to disk as well as the final status report.
• A web worker is used to dynamically show the import progress and the final status report.

Handling the import asynchronously and writing the status report to disk would make the latest import results available to the user even if he left the page to check back later. It would also make it possible to revise reports for automated imports invoked by cron jobs.

As usual, comments by @brendo are very welcome.

[nitriques]

A web worker is used to dynamically show the import progress and the final status report.

This is hard to do since web workers do not have access to the DOM and everything would need to get sent via message passing.

I would rather use ajax polling... Much simpler to code and maintain.

The overall, very good idea.