Custom variant type for packetheader

Summary:
Make a custom variant type for PacketHeader. By not relying on boost::variant
this reduces the code size of the implementation.

This uses a combination of a union type as well as a enum type to emulate a variant

Reviewed By: yangchi

Differential Revision: D17187589

fbshipit-source-id: 00c2b9b8dd3f3e73af766d84888b13b9d867165a

quic/loss/QuicLossFunctions.cpp

@@ -126,9 +126,7 @@ void markPacketLoss(
           if (processed) {
             return;
           }
-          auto protectionType = folly::variant_match(
-              packet.header,
-              [](auto& header) { return header.getProtectionType(); });
+          auto protectionType = packet.header.getProtectionType();
           auto encryptionLevel =
               protectionTypeToEncryptionLevel(protectionType);
           auto cryptoStream =