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/handlers_data_test.go

@@ -443,7 +443,7 @@ func TestHandleMsgMhfLoaddata_Integration(t *testing.T) {
 	s := createTestSession(mock)
 	s.charID = charID
 	SetTestDB(s.server, db)
-	s.server.userBinaryParts = make(map[userBinaryPartID][]byte)
+	s.server.userBinary = NewUserBinaryStore()
 
 	pkt := &mhfpacket.MsgMhfLoaddata{
 		AckHandle: 5678,