The server no longer stalls when many cards land

When many cards landed one after another, Obeya's server froze for half a second at a time: pages got no answer and agents went unheard. Every change on the canvas rebuilt the whole canvas view, dozens of times in a row. Now everything that reacts to changes hears of them once a run of changes is through, cards are read back without rebuilding the rest of the view, and an archived card is looked up without building the whole archive. A landing of 30 cards on a large canvas now holds the server up for about a tenth of a second instead of more than half a second.