message: Defer addBufferMeta/EOM sending to onHeaderBytesGenerated callback.
  Summary: This allows for the DSRRequestSender to have the number of header bytes available before any buffer metas are added to the transaction.
diff: @@ -241,18 +241,12 @@ class MockQuicSocket : public QuicSocket {
  (StreamId, const BufferMeta&, bool, ByteEventCallback*));
  MOCK_METHOD(
  WriteResult,
  - setDSRPacketizationRequestSenderRef,
  - (StreamId, const std::unique_ptr<DSRPacketizationRequestSender>&));
  - WriteResult setDSRPacketizationRequestSender(
  - StreamId streamId,
  - std::unique_ptr<DSRPacketizationRequestSender> sender) override {
  - return setDSRPacketizationRequestSenderRef(streamId, sender);
  - }
  + setDSRPacketizationRequestSender,
  + (StreamId, std::unique_ptr<DSRPacketizationRequestSender>));
  MOCK_METHOD(
  (folly::Expected<folly::Unit, LocalErrorCode>),
  registerDeliveryCallback,
  (StreamId, uint64_t, ByteEventCallback*));
  -
  MOCK_METHOD(folly::Optional<LocalErrorCode>, shutdownWrite, (StreamId));
  MOCK_METHOD(
  (folly::Expected<folly::Unit, LocalErrorCode>),
  