dynamically adjust readahead, for nearly sequential download

src/server/torr/storage/memcache/Cache.go

@@ -211,3 +211,10 @@ func (c *Cache) ReadersLen() int {
 	}
 	return len(c.readers)
 }
+
+func (c *Cache) AdjustRA(readahead int64) {
+	for r, _ := range c.readers {
+		r.SetReadahead(readahead)
+	}
+}
+