Don't recheck connected clients in isbanned() (and other isbanned() dos mitigations) (#48583)

dos mitigation that likely won't fully work but let's see anyways.

backstory:

byond's ban system will automatically ban world/Topic flooders. Sadly updating byond's internal ban list causes isbanned() to get called on all connected clients. Also sadly: it does this every time it sees the world/Topic flood on the same ip, causing every connected client gets checked repeatedly for every flood message world/Topic sees.

code/modules/admin/IsBanned.dm

@@ -13,10 +13,17 @@
 			return FALSE
 		log_access("Failed Login (invalid data): [key] [address]-[computer_id]")
 		return list("reason"="invalid login data", "desc"="Error: Could not check ban status, Please try again. Error message: Your computer provided invalid or blank information to the server on connection (byond username, IP, and Computer ID.) Provided information for reference: Username:'[key]' IP:'[address]' Computer ID:'[computer_id]'. (If you continue to get this error, please restart byond or contact byond support.)")
-
+	
+	if (type == "world")
+		return ..() //shunt world topic banchecks to purely to byond's internal ban system
+		
 	var/admin = FALSE
 	var/ckey = ckey(key)
-
+	
+	var/client/C = GLOB.directory[ckey]
+	if (C && ckey == C.ckey && computer_id == C.computer_id && address == C.address)
+		return //don't recheck connected clients.
+		
 	//IsBanned can get re-called on a user in certain situations, this prevents that leading to repeated messages to admins.
 	var/static/list/checkedckeys = list()
 	//magic voodo to check for a key in a list while also adding that key to the list without having to do two associated lookups
@@ -25,8 +32,6 @@
 	if(GLOB.admin_datums[ckey] || GLOB.deadmins[ckey])
 		admin = TRUE
 
-	var/client/C = GLOB.directory[ckey]
-
 
 	//Whitelist
 	if(!real_bans_only && !C && CONFIG_GET(flag/usewhitelist))