Implemented per world setting to keep the spawn in memory or not.

src/main/java/net/minecraft/server/World.java

@@ -83,6 +83,7 @@ public class World implements IBlockAccess {
     // CraftBukkit start
     private final CraftWorld world;
     public boolean pvpMode;
+    public boolean keepSpawnInMemory = true;
     public ChunkGenerator generator;
     Chunk lastChunkAccessed;
     int lastXAccessed = Integer.MIN_VALUE;