define a quic.TokenStore interface and provide a LRU implementation

interface.go

@@ -27,6 +27,23 @@ type Token struct {
 	SentTime     time.Time
 }
 
+// A ClientToken is a token received by the client.
+// It can be used to skip address validation on future connection attempts.
+type ClientToken struct {
+	data []byte
+}
+
+type TokenStore interface {
+	// Pop searches for a ClientToken associated with the given key.
+	// Since tokens are not supposed to be reused, it must remove the token from the cache.
+	// It returns nil when no token is found.
+	Pop(key string) (token *ClientToken)
+
+	// Put adds a token to the cache with the given key. It might get called
+	// multiple times in a connection.
+	Put(key string, token *ClientToken)
+}
+
 // An ErrorCode is an application-defined error code.
 // Valid values range between 0 and MAX_UINT62.
 type ErrorCode = protocol.ApplicationErrorCode