This resolves #702

* Made GetOrCreateUser always call AddRef and added check to PRESENCE_UPDATE to avoid readding users who have been removed from guilds

* Removed AddRef as per dev guild discussion

* Replace Where.FirstOrDefault with FirstOrDefault

* Replace Where+FirstOrDefault in ClientHelper

The previous commit prevents any connections, since the initial presence update is sent while the client is still in the 'connecting' state, rather than the 'connected' state.

This resolves the original issue by preventing a nullref, and the more recent issue by only throwing a detailed exception when the CurrentUser is null (the client isn't logged in).

This prevents a later, less detailed nullref, when attempting to set the CurrentUser's presence data.

This also removes a redundant CurrentUser assignment in the SetGameAsync method, since this will be set later on in the SendStatusAsync method.

This should resolve a plethora of relates issues, including user downloading not working (#542), possibly #531

Conflicts:
	src/Discord.Net.WebSocket/DiscordSocketClient.cs