Add autogenerate-to-visibilitylimits option to allow automatic priming/generating of chunks within defined visibility area

Avoid rendering tiles with nothing but empty chunks
Limit size of hash-file cache, and move hash files to world-specific tile directories

src/main/java/org/dynmap/utils/MapChunkCache.java

@@ -37,6 +37,10 @@ public interface MapChunkCache {
      * Test if done loading
      */
     boolean isDoneLoading();
+    /**
+     * Test if all empty blocks
+     */
+    boolean isEmpty();
     /**
      * Unload chunks
      */
@@ -87,4 +91,8 @@ public interface MapChunkCache {
      * Coordinates are block coordinates
      */
     public void setVisibleRange(VisibilityLimit limit);
+    /**
+     * Set autogenerate - must be done after at least one visible range has been set
+     */
+    public void setAutoGenerateVisbileRanges(boolean do_generate);
 }