Make unbounded channels size warning exact (part 1) (#13490)

* Replace `futures-channel` with `async-channel` in `out_events`

* Apply suggestions from code review

Co-authored-by: Koute <[email protected]>

* Also print the backtrace of `send()` call

* Switch from `backtrace` crate to `std::backtrace`

* Remove outdated `backtrace` dependency

* Remove `backtrace` from `Cargo.lock`

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Co-authored-by: Koute <[email protected]>