extend export command to show tombstone + change output format to CSV

merging https://github.com/chrislusf/seaweedfs/pull/610 and add "-limit" option

weed/storage/needle.go

@@ -258,3 +258,7 @@ func ParseNeedleIdCookie(key_hash_string string) (NeedleId, Cookie, error) {
 	}
 	return needleId, cookie, nil
 }
+
+func (n *Needle) LastModifiedString() string {
+	return time.Unix(int64(n.LastModified), 0).Format("2006-01-02T15:04:05")
+}

weed/storage/volume_read_write.go

@@ -217,7 +217,11 @@ func ScanVolumeFile(dirname string, collection string, id VolumeId,
 				glog.V(4).Infof("Adjusting n.Size %d=>0 rest:%d=>%d %+v", oldSize, oldRest, rest, n)
 			}
 		}
-		if err = visitNeedle(n, offset); err != nil {
+		err = visitNeedle(n, offset)
+		if err == io.EOF {
+			return nil
+		}
+		if err != nil {
 			glog.V(0).Infof("visit needle error: %v", err)
 		}
 		offset += NeedleEntrySize + rest