clear pieces on cancel

core/src/main/groovy/com/muwire/core/download/Downloader.groovy

@@ -180,6 +180,7 @@ public class Downloader {
             piecesFile.delete()
         }
         incompleteFile.delete()
+        pieces.clearAll()
     }
     
     void stop() {

core/src/main/groovy/com/muwire/core/download/Pieces.groovy

@@ -78,4 +78,9 @@ class Pieces {
     synchronized boolean isDownloaded(int piece) {
         done.get(piece)
     }
+    
+    synchronized void clearAll() {
+        done.clear()
+        claimed.clear()
+    }
 }