title: Huge amount of Placemarks on MKMapView
body: <p>i have a application, which is connected to a webservice, and it is getting realtime informations about something happened at some places.
  The Information have a timestamp and Lat/Lon coordinates.
  I store each of that datasets into a <code>NSMutableArray</code>. In some cases, there can be several thounsands (50.000) of datasets in it.
  That all is working great. But now i want to have a <code>MKMap</code> on which i place each dataset as a small dot (6px diameter) on the desired location.
  I have a function, that is called if a new message is placed in the array. Here i can put the new point to the map.</p>
  
  <p>My question is now, when i use annotaions, the <code>MKMap</code> will be unusable due to the high amount of annotations. Every two seconds i want to redraw the points in the view area. I do this, because i want to change the colors of the points. Color from Yellow to brown depends on the age of the message. </p>
  
  <p>What is the best method to do this ? What about a Core Graphics layer over the <code>MKMap</code> ?
  And what about if i drag the map, or if i change the zoomlevel? I think, I need a redraw of the map then.</p>