* AbstractSpot and AbstractEdge both extend PoolObject
* AbstractSpotPool and AbstractEdgePool both extends Pool
  (Pool is a new abstract class, the previous Pool is now named MemPool)
* PoolObjectList< O extends PoolObject > is a lightweight List< O > that
  is backed by a TIntList index list.
* PoolObjectSet< O extends PoolObject > is a lightweight Set< O > that
  is backed by a TIntSet index set.
small javadoc change
move common parts of AbstractEdgePool and AbstractSpotPool to superclass Pool
remove unused code