Address (most) clippy warnings

[michaelklishin]

A lot of cargo clippy warnings are unused variables and trivial things like that. Should I submit a PR that squashes as many of them as possible within a certain time box (say, a couple of hours)?

[michaelklishin]

Now I understand why: most warnings come from conditionally compiled code like this:

Err(err) => {
  #[cfg(feature="traces")]
  error!("cancel callback error on channel {}, cause: '{}'.", self.channel, err);
}

[gftea]

If compiles with all features, it should be no warning

[michaelklishin]

If compiles with all features, it should be no warning

Fair enough.