refactor(channelserver): extract QuestCache from Server struct

The quest cache fields (lock, data map, expiry map) were spread across
Server with manual lock management. The old read path also missed the
RLock entirely, creating a data race. Encapsulating in a dedicated type
fixes the race and reduces Server's field count by 2.

server/channelserver/sys_channel_server_test.go

@@ -58,8 +58,7 @@ func createTestServer() *Server {
 		sessions:       make(map[net.Conn]*Session),
 		stages:         make(map[string]*Stage),
 		semaphore:      make(map[string]*Semaphore),
-		questCacheData: make(map[int][]byte),
-		questCacheTime: make(map[int]time.Time),
+		questCache: NewQuestCache(0),
 		erupeConfig: &cfg.Config{
 			DebugOptions: cfg.DebugOptions{
 				LogOutboundMessages: false,