fetch: replace futures-timer with tokio-timer

Currently the coverage build fails because `futures-timer` fails to compile with
`-C link-dead-code`. This issue has been reported to `futures-timer`
(https://github.com/alexcrichton/futures-timer/issues/2) but has remained unsolved
for months. It should be fixed by rustc eventually
(https://github.com/rust-lang/rust/issues/45629).