First bits of the Context overhaul.

Reintroduces separate contexts for users, channels, connections (now split into sessions and connections) and user-channel associations.
It builds which is as much assurance as I can give about the stability of this commit, but its also the bare minimum of what i like to commit sooooo
A lot of things still need to be broadcast through events throughout the application in order to keep states consistent but we'll cross that bridge when we get to it.
I really need to stop using that phrase thingy, I'm overusing it.

SharpChat.SQLite/SQLiteMessageStorage.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging;
 using SharpChat.Data;
 using SharpChat.Messages;
+using SharpChat.Users;
 using System.Data;
 using System.Data.Common;
 using System.Data.SQLite;