Let DiscordVoiceConnections be reused

[Francessco121]

At the moment, DiscordVoiceConnections become "invalid" if anything goes wrong or if they disconnect for any reason. This makes the class a bit clunky to use.

This is a proposal to:

• Remove the concept of voice connection invalidation
• Let "fatal" errors simply disconnect the connection
• Add better automatic reconnection where possible

With this change, DiscordVoiceConnections would be able to simply be created for each guild and live for the entire duration of the associated bridge. This massively simplifies usage, but would be a pretty large breaking change.

Note: The only "invalid" state should be when the class is disposed.