Call exitWhenFinished on FileCleaningTracker

Without calling this, FileCleaningTracker threads will never terminate. When
running oxgarage as a web service, this is a memory leak.

Author: egh

src/main/java/pl/psnc/dl/ege/utils/DataBuffer.java

@@ -244,6 +244,7 @@ public void clear(boolean forceDelete)
 			}
 		}
 		items = new HashMap<String,Item>();
+		tracker.exitWhenFinished();
 	}
 
 	public String getTemporaryDir(){