Tokio v0.2.0

A major breaking change. Most implementation and APIs have changed one way or
another. This changelog entry contains a highlight

Changed

• APIs are updated to use async / await.
• most tokio-* crates are collapsed into this crate.
• Scheduler is rewritten.
• tokio::spawn returns a JoinHandle.
• A single I/O / timer is used per runtime.
• I/O driver uses a concurrent slab for allocating state.
• components are made available via feature flag.
• Use bytes 0.5
• tokio::codec is moved to tokio-util.

Removed

• Standalone timer and net drivers are removed, use Runtime instead
• current_thread runtime is removed, use tokio::runtime::Runtime with
  basic_scheduler instead.

v0.2.0-alpha.5

Changed

• sync: rename Lock -> Mutex and make it more like std::sync::Mutex (#1573).
• time: rename sleep to delay_for (#1518).

Fixed

• executor: shutdown blocking pool threads when idle (#1562, #1514).
• fs: propagate flush for stdout / stderr. (#1528).
• net: API documentation generation (#1575).

Added

• io: bring back generic split for AsyncRead + AsyncWrite (#1521).
• io: enable buffering both reads and writes on the same type (#1558).
• process: platform specific Command methods (#1516).
• process: implement From<std::process::Command> for Command (#1513).
• tls: TlsStream::get_ref and TlsStream::get_mut (#1537).