Update tokio requirement from 0.2 to 0.3

[dependabot]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.3.0

This represents a 1.0 beta release. APIs are polished and future-proofed. APIs not included for 1.0 stabilization have been removed.

Biggest changes are:

• I/O driver internal rewrite. The windows implementation includes significant changes.
• Runtime API is polished, especially with how it interacts with feature flag combinations.
• Feature flags are simplified
  • rt-core and rt-util are combined to rt
  • rt-threaded is renamed to rt-multi-thread to match builder API
  • tcp, udp, uds, dns are combied to net.
  • parking_lot is included with full

Changes

• meta: Minimum supported Rust version is now 1.45.
• io: AsyncRead trait now takes ReadBuf in order to safely handle reading into uninitialized memory (#2758).
• io: Internal I/O driver storage is now able to compact (#2757).
• rt: Runtime::block_on now takes &self (#2782).
• sync: watch reworked to decouple receiving a change notification from receiving the value (#2814, #2806).
• sync: Notify::notify is renamed to notify_one (#2822).
• process: Child::kill is now an async fn that cleans zombies (#2823).
• sync: use const fn constructors as possible (#2833, #2790)
• signal: reduce cross-thread notification (#2835).
• net: tcp,udp,uds types support operations with &self (#2828, #2919, #2934).
• sync: blocking mpsc channel supports send with &self (#2861).
• time: rename delay_for and delay_until to sleep and sleep_until (#2826).
• io: upgrade to mio 0.7 (#2893).
• io: AsyncSeek trait is tweaked (#2885).
• fs: File operations take &self (#2930).
• rt: runtime API, and #[tokio::main] macro polish (#2876)
• rt: Runtime::enter uses an RAII guard instead of a closure (#2954).

Added

• sync: map function to lock guards (#2445).
• sync: blocking_recv and blocking_send fns to mpsc for use outside of Tokio (#2685).
• rt: Builder::thread_name_fn for configuring thread names (#1921).
• fs: impl FromRawFd and FromRawHandle for File (#2792).
• process: Child::wait and Child::try_wait (#2796).
• rt: support configuring thread keep-alive duration (#2809).
• rt: task::JoinHandle::abort forcibly cancels a spawned task (#2474).
• sync: RwLock write guard to read guard downgrading (#2733).
• net: add poll_* functions that take &self to all net types (#2845)
• sync: get_mut() for Mutex, RwLock (#2856).
• sync: mpsc::Sender::closed() waits for Receiver half to close (#2840).
• sync: mpsc::Sender::is_closed() returns true if Receiver half is closed (#2726).
• stream: iter and iter_mut to StreamMap (#2890).

... (truncated)

Commits

• 12f1dff chore: prepare for v0.3.0 release (#2960)
• 871289b ci: run clippy on MSRV (#2962)
• 30b40ef rt: update docs for 0.3 changes (#2956)
• 22fa883 rt: tweak spawn_blocking docs (#2955)
• 00b6127 rt: switch enter to an RAII guard (#2954)
• a249421 process: update docs regarding zombie processes (#2952)
• 1923350 meta: combine net and dns, use parking_lot (#2951)
• c90681b rt: simplify rt-* features (#2949)
• 24d0a0c chore: refactor runtime driver usage of Either (#2918)
• 07802b2 rt: worker_threads must be non-zero (#2947)
• Additional commits viewable in compare view

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[mxinden]

Not yet ready to be merged.

[dependabot]

A newer version of tokio exists, but since this PR has been edited by someone other than Dependabot I haven't updated it. You'll get a PR for the updated version as normal once this PR is merged.