refactor(mhfpacket): rename 15 Unk fields with identified meanings

Replace unknown field names with descriptive names based on handler
logic analysis, switch dispatch patterns, DB query context, and
inline comments:

- ObjectHandleID, IsQuest, ItemIDCount, MaxCount, TokenLength,
  FormatVersion, LogoutType (high confidence from comments/constants)
- QueryType, DataType, MissionIndex, CheckOnly, RequestType,
  ExchangeType, TournamentID (confirmed by handler switch/if usage)

Also fix MsgSysLogout.Build calling ReadUint8 instead of WriteUint8.

server/channelserver/handlers_mercenary.go

@@ -148,7 +148,7 @@ func handleMsgMhfSaveHunterNavi(s *Session, p mhfpacket.MHFPacket) {
 
 func handleMsgMhfMercenaryHuntdata(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfMercenaryHuntdata)
-	if pkt.Unk0 == 1 {
+	if pkt.RequestType == 1 {
 		// Format:
 		// uint8 Hunts
 		// struct Hunt