Store session start timestamp (when demo recording started) in header
Use connection Guid instead of dictionary in player lookup (to avoid players reconnecting causing issues)
- change DemoPlayerInfo.SteamId -> Guid (ulong technically)
Change path to `server-demos/{serverid}/sessions/{sessionid}/{chunkid}.sdem` (to make it easier to navigate in azure)
Fix compressedSize in demo header being incorrect (FileStream.Length > FileStream.Position)