Update tokio requirement from 0.1.11 to 0.3.7 #2542

Job	Run time
cargo bench	49s
cargo +1.36 build (futures-{core, io, sink})	1m 25s
cargo +stable build	25s
cargo clippy	24s
cargo +beta build	29s
cargo doc	19s
cargo hack check --feature-powerset	2m 28s
cargo +nightly build	29s
cargo fmt	3s
cargo minimal-versions build	29s
cargo miri test	7m 53s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	1m 6s
cargo build --target thumbv6m-none-eabi	26s
cargo test -Z sanitizer=memory	51s
cargo test (ubuntu-latest)	22s
cargo +1.56 build	1m 34s
cargo test -Z sanitizer=thread	51s
cargo test (macos-latest)	40s
cargo test (windows-latest)	47s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	1m 5s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	51s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	46s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	1m 16s
	26m 13s

Provide feedback