FSStorage: free written piece buffers

lib/fs-storage.js

@@ -160,6 +160,7 @@ FSStorage.prototype._onPieceDone = function (piece) {
   var writeToNextFile = function (err) {
     if (err) return self.emit('error', err)
     if (i >= end) {
+      delete piece.buffer
       return cb()
     }