refactor(channelserver): extract UserBinaryStore and MinidataStore

The userBinary and minidata maps with their locks were spread across
Server as raw fields with manual lock management. Cross-channel session
searches also required acquiring nested locks (server lock + binary
lock). Encapsulating in dedicated types eliminates the nested locking
and reduces Server's field count by 4.

server/channelserver/session_lifecycle_integration_test.go

@@ -583,8 +583,8 @@ func createTestServerWithDB(t *testing.T, db *sqlx.DB) *Server {
 		db:              db,
 		sessions:        make(map[net.Conn]*Session),
 		stages:          make(map[string]*Stage),
-		userBinaryParts: make(map[userBinaryPartID][]byte),
-		minidataParts:   make(map[uint32][]byte),
+		userBinary: NewUserBinaryStore(),
+		minidata:   NewMinidataStore(),
 		semaphore:       make(map[string]*Semaphore),
 		erupeConfig: &cfg.Config{
 			RealClientMode: cfg.ZZ,