Guilds voice_states cache sometimes corrupts itself about disco

[kyriog]

Okay, I investigated a little bit about this on my side.
I've found an easy way to reproduce this:

1. Open Discord twice with the same account (eg. PC + navigator, PC + Smartphone…)
2. On Device 1, connect to any vocal channel.
3. It will trigger a VoiceStateUpdate with session_id A (so Device 1 = Session ID A), and it'll be added to state.voice_states and state.guild[guild_id].voice_states
4. Without disconnecting device 1, on Device 2, connect to any vocal channel from the same server (Same channel as device 1 or not, it doesn't matter).
5. It will trigger a VoiceStateUpdate with session_id B (Device 2 = Session ID B), which will be added to the two same hashmaps.
6. No VoiceStateUpdate will be triggered with the old disconnected session A.
   => The self.voice_states now contains a wrong entry. It will only be removed if the user connects using device 1 (session A) again later (or by restarting the bot).

I think we should use user_id rather than session_id for voice_states keys, as a same user can't be vocally connected twice from two devices.

I'll try to make a PR to fix this issue.

[kyriog]

This is a "quick&easy" fix which does not broke the voice_states management.

imo, we should completly replace session_id key by user_id but as it could broke some bots, and as I don't know if you're agree with me, I made this first.