Migrate calls from Players.Length to ActivePlayers

As pointed out by @QuiCM, TShock.Players is actually an array and not a
smarter collection, so length will return the total collection size and
not the active players. An earlier commit was added that gives TSPlayer
an ICollection<TSPlayer> that contains only active players. This is now
the basis of determining the number of active players on the server.

TShockAPI/StatTracker.cs

@@ -144,7 +144,7 @@ namespace TShockAPI
 			return new JsonData()
 			{
 				port = Terraria.Netplay.ListenPort,
-				currentPlayers = TShock.Players.Length,
+				currentPlayers = TSPlayer.ActivePlayers.Count,
 				maxPlayers = TShock.Config.MaxSlots,
 				systemRam = GetTotalSystemRam(ServerApi.RunningMono),
 				version = TShock.VersionNum.ToString(),