Sort markers by display time

Sorted the markers to be added to the map, in preparation of eventually staggering the
addition of markers from one big addition of all markers of each call, to be incrementally
added over time.  This will reduce the cycles needed to loop through each marker on each
frame of the animation.
Markers are added to the map in chunks

Previously after a single AJAX call for marker data, all the markers would be added to the
map that were received in one big chunk.  All these markers are then looped over in the
update loop each frame, even though for big groups of markers, most of them are not to
be displayed yet.  So instead the received markers are added to the map in chunks of, for
now, 10.