Arrows Not Despawning

Found 2 Skeleton Spawners. Made a Mob Trap with water.
Skeletons shoot arrows. All arrows shot into cobblestone stay forever. Water will remove them. They do not despawn however.
Step 1. Create Server.
Step 2. Find Skeleton Spawner
Step 3. Let Spawner Spawned Skeletons shoot Cobblestone (Or any other block from what I can tell)
Step 4. Watch Entity Count Rise to unacceptable levels.