Provide names and pools when creating MiniYaml.

- Rename the filename parameter to name and make it mandatory. Review all callers and ensure a useful string is provided as input, to ensure sufficient context is included for logging and debugging. This can be a filename, url, or any arbitrary text so include whatever context seems reasonable.
- When several MiniYamls are created that have similar content, provide a shared string pool. This allows strings that are common between all the yaml to be shared, reducing long term memory usage. We also change the pool from a dictionary to a set. Originally a Dictionary had to be used so we could call TryGetValue to get a reference to the pooled string. Now that more recent versions of dotnet provide a TryGetValue on HashSet, we can use a set directly without the memory wasted by having to store both keys and values in a dictionary.

OpenRA.Game/Network/Session.cs

@@ -41,13 +41,13 @@ namespace OpenRA.Network
 			return null;
 		}
 
-		public static Session Deserialize(string data)
+		public static Session Deserialize(string data, string name)
 		{
 			try
 			{
 				var session = new Session();
 
-				var nodes = MiniYaml.FromString(data);
+				var nodes = MiniYaml.FromString(data, name);
 				foreach (var node in nodes)
 				{
 					var strings = node.Key.Split('@');