Remove /users/@me call for socket and rework sharded client a bit

[SubZero0]

Summary

The current user is sent on READY and there's no need to do a rest request for it (much less N of them if using a sharded client), so this call was removed from the socket client.
In a few times some shards that werent the first one would be ready faster and DiscordShardedClient.CurrentUser could be null, now this property will get the first not null CurrentUser from any shard.

Changes

• Removed /users/@me rest request for socket client
• DiscordShardedClient.Rest might return null instead of throwing an exception if it wasnt initialized yet
• DiscordShardedClient.CurrentUser will get the first not null CurrentUser from any shard instead of just the "primary"