Implement poller for slow AJAX handlers

[JoosuaKoskinen]

Is your feature request related to a problem? Please describe.
Some handlers are slow on bigger sites and cause executing too slow errors.

The errors are the only issue, the features can't really be made faster.

Describe the solution you'd like
Slow handlers should implement the new AJAX poller feature so slow features are ran on background and checked on every X seconds.