Update tokio requirement from 0.1.12 to 0.2.17

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio 0.2.17

This release fixes a bug in the threaded scheduler that could result in panics under load (see #2382). Additionally, the default number of worker threads now uses the logical CPU count, so it will now respect scheduler affinity and cgroups CPU quotas.

Fixes

• rt: bug in work-stealing queue (#2387)

Changes

• rt: threadpool uses logical CPU count instead of physical by default (#2391)

Commits

• 3137c6f chore: prepare to release 0.2.17 (#2392)
• d294c99 Use logical CPUs instead of physical by default (#2391)
• 58ba45a rt: fix bug in work-stealing queue (#2387)
• de8326a doc: Sort methods on mpsc::Sender in doc (#2379)
• d65bf38 doc: add error explanation for UnboundedSender::send() (#2372)
• 7c1bc46 test: add Send/Sync tests for all async fns (#2377)
• d883ac0 chore: prepare tokio 0.2.16 release
• 1121a8e sync: ensure Mutex, RwLock, and Semaphore futures are Send + Sync (#2375)
• 6fa40b6 doc: Fix readme link (#2370)
• e10471d io: Add StreamReader (#2052)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #113.