Fix config file access errors when starting up multiple TShock servers at once

Use case would be multi-server networks. This commit ensures that sscconfig.json is read with FileShare.Read enabled and stops writing to config.json and sscconfig.json on server load immediately after reading it if the file already exists, which is a no-op since neither of the config files have changed at that point.

TShockAPI/ServerSideCharacters/ServerSideConfig.cs

@@ -48,7 +48,8 @@ namespace TShockAPI.ServerSideCharacters
 
 		public static ServerSideConfig Read(string path)
 		{
-			using (var reader = new StreamReader(path))
+			using (var fileStream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read))
+			using (var reader = new StreamReader(fileStream))
 			{
 				string txt = reader.ReadToEnd();
 				var config = JsonConvert.DeserializeObject<ServerSideConfig>(txt);