message: support buf clone in BufQueue
  Summary: Add a method to allow clone the IOBuf held by the queue.
diff: @@ -51,6 +51,10 @@ class BufQueue {
  return std::move(chain_);
  }
  + Buf clone() const {
  + return chain_ ? chain_->clone() : nullptr;
  + }
  +
  const folly::IOBuf* front() const {
  return chain_.get();
  }
  