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.Mods.Common/ServerTraits/LobbyCommands.cs

@@ -1270,7 +1270,7 @@ namespace OpenRA.Mods.Common.Server
 
 				try
 				{
-					server.LobbyInfo = Session.Deserialize(s);
+					server.LobbyInfo = Session.Deserialize(s, nameof(SyncLobby));
 					server.SyncLobbyInfo();
 				}
 				catch (Exception)