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/channel_registry_test.go

@@ -14,7 +14,7 @@ func createTestChannels(count int) []*Server {
 		s.IP = "10.0.0.1"
 		s.Port = uint16(54001 + i)
 		s.GlobalID = "0101"
-		s.userBinaryParts = make(map[userBinaryPartID][]byte)
+		s.userBinary = NewUserBinaryStore()
 		channels[i] = s
 	}
 	return channels