unwatch files when removing them from the cache

lib/mbtiles.js

@@ -83,6 +83,7 @@ MBTiles.prototype._open = function(uri) {
         mbtiles._stat = stat;
         fs.watchFile(mbtiles.filename, { interval: 1000 }, function(cur, prev) {
             if (cur.mtime != prev.mtime) {
+                fs.unwatchFile(mbtiles.filename);
                 delete cache[uri.pathname];
             }
         });