integration starts should be delayed

currently, the integrator simply polls every seven minutes, and will start an integration as soon as it sees a single staged change.
this is annoying, because it may start an integration in the middle of somebody staging a series of commits, which - given QTQAINFRA-444 - leads to annoyance.

therefore, the integrator should look at the timestamp of the last staging action, and hold off integration if it happened very recently.

there is also another thing this delay can be used for: to avoid that integration runs are started for a single change, thus wasting lots of resources (though it might be interesting to be able to mark changes as urgent, so they do not get delayed).

i suggest that the delay time starts at two hours, and gets reduced with the number of staged commits, down to two minutes when there are 20 commits staged already.