v3: voice: interrupted connection if user moves the bot to another channel

[diamondburned]

This issue is a follow-up on #238 and its PR #241.

Right now, voice is implemented so that it is reconnected when either a VoiceServerUpdateEvent or a VoiceStateUpdateEvent arrives from the main gateway. This is actually incorrect, and the bot should wait for both before reconnecting.

Since it only handles one of those events, at the point of reconnection, the information given to the server is invalid, so the connection cannot be resumed and everything is interrupted.