Update dispatching is broken and racy

[emersion]

Broken

The way channels are used makes it so updates are only sent to the first client. See https://github.com/emersion/go-imap/pull/336

Racy

We read from the connection context:

https://github.com/emersion/go-imap/blob/3bc38e360e3c75d52e3e2c60f3597369154ec4b4/server/server.go#L331

However there's an assumption that only the connection's goroutine will access it.

[foxcpp]

Issue is no longer relevant since #374 is a thing?

[emersion]

go-imap v2 is not released yet, let's wait before closing this.

[foxcpp]

Yeah, just making a note that there is no need to spend a lot of effort fixing troubles with v1 code.