change HQDownstreamSession destructor from private to protected

Summary:
- Per https://fburl.com/code/lb8pgvwb, to use DelayedDestruction, the destructor can be either private or protected
- HQDownstreamSession destructor is private, which prevents users from subclassing it.
- Let's change it to "protected"

Reviewed By: lnicco

Differential Revision: D71357167

fbshipit-source-id: aeeb97c6a80dedd6345db6c03bfe2e4c882a28eb

Author: Siming Li

proxygen/lib/http/session/HQDownstreamSession.h

@@ -84,7 +84,7 @@ class HQDownstreamSession : public HQSession {
   folly::Optional<HTTPHeaders> getExtraHeaders(
       const HTTPMessage& haeders, quic::StreamId streamId) override;
 
- private:
+ protected:
   ~HQDownstreamSession() override {
     CHECK_EQ(getNumStreams(), 0);
   }