Joining a channel returns a future which fires on receipt of two messages from discord (by locally storing a channel). However, joining this same channel again after a success returns only *one* such message, causing the command to hang until another join fires or the channel is left. This alters internal behaviour to correctly cancel an in-progress connection attempt, or return success with known data if such a connection is present.
This introduces a breaking change on `Call::update_state` to include the target `ChannelId`. The reason for this is that although the `ChannelId` of a target channel was being stored, server admins may move or kick a bot from its voice channel. This changes the true channel, and may accidentally trigger a "double join" elsewhere.
This fix was tested by using an example to have a bot join its channel twice, to do so in a channel it had been moved to, and to move from a channel it had been moved to.
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