Improve sending notifications process

[aabounegm]

Add proper debounce to the new_arrivals notification https://github.com/illright/innopoints-backend/blob/1303c953823820382066b0202b96d20ae8971910/innopoints/views/product.py#L121

Can easily be solved after solving the issue of sending notifications in a separate thread (using threading.Timer)
Both tasks are in the scope of this issue.

[illright]

Actually, this APScheduler thing seems a little more mature than that, with persistence options (though I'm not sure if we really need to protect against restarts for this type of notification).

[aabounegm]

It feels like an overkill while threading is built-in. The only issue that remained was to understand how Flask threading works to not break things by mistake.

[illright]

That's the thing, I really don't think we should be messing with Flask's thread management (not to mention workers) when established solutions are readily available