Explore optimizations of annotations rendering and OpenLayers

[fedorov]

Follow up on #330

1. evaluate the effort that will be needed to improve organization and maintainability of the code base, which in particular would unblock the main thread while performing time-consuming operations (fetching the data from the server, creating annotations, rendering)
2. could explore rendering of the polygons outside of OpenLayers and passing the result as overlay to OpenLayers (need to make sure we do not completely eliminate the possibility of allowing editing polygons in the future - we just need to be able to optimize for rendering in read-only)
3. look if there is something new in the latest OpenLayers (we are close to latest) - probably do this first, since the latest 1.7.0 release mentions some relevant improvements
4. would it help, would it be possible to reuse instances for the annotations?