Allow rate-limiting the DirectoryScanner via time-slicing

When the inodes and directory entries for HDFS block and meta files are not in the cache, the DirectoryScanner can create enormous disk load via generating many seeks. This can effectively make the disks unusable for other purposes. We should allow rate-limiting the DirectoryScanner via time-slicing, so that it only runs every 100 ms out of 1 s (or whatever other percentage we configure.)