SImpleDistanceBased: move items into Clusters that are closer.
Have QuadItem implement Cluster, so no allocation of a Cluster object is necessary for a single marker.
Move quantization of zoom and caching out to a decorator Algorithm class.
interface Cluster.getItems() now returns a Collection, not a Set (HashSet performs a lot of allocations).
Remove deprecated use of projection.Point
Remove use of deprecated projection.Point
PointQuadTree returns a Collection, instead of a Set.
Add an optimization - Bounds.contains(Bounds)
Add a decorator Algorithm that quantizes zoom level, fetches and caches (with LruCache) the clusters for the two adjacent zoom levels.