This issue was especially visible in tests. the `@debounce(100)` was not cancelled when changing channel which was causing 404s as we were trying to load messages on a channel which was deleted as the channel has been destroyed at the end of the test.
This is still not a perfect solution, as we can only cancel the start of `fetchMessages`, but we can't cancel the actual `chatApi.channel` request which result can potentially happens after channel changed, which we try to mitigate with various checks on to ensure visible channel == loaded messages channel.
This commit also tries to make handler naming and cancelling more consistent.
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