send PATH_RESPONSEs on the same path (#4991)

* make it possible to pack path probes with multiple frames

* simplify function signature of pathManager.HandlePacket

* simplify connection short header packet handling logic

No functional change expected.

* make server send PATH_RESPONSEs on the same path

This makes sure that we’re actually testing for return routability.

integrationtests/self/connection_migration_test.go

@@ -18,7 +18,7 @@ import (
 )
 
 func TestConnectionMigration(t *testing.T) {
-	ln, err := quic.ListenAddr("localhost:0", tlsConfig, getQuicConfig(nil))
+	ln, err := quic.ListenAddr("localhost:0", getTLSConfig(), getQuicConfig(nil))
 	require.NoError(t, err)
 	defer ln.Close()