chore: prepare for v0.2.0-alpha.1 release (tokio-rs#1410)

Bump `tokio-sync` to 0.1.6 (tokio-rs#1123)

Bump `tokio` version to v0.1.21. (tokio-rs#1113)

Release tokio v0.1.20, tokio-timer v0.2.21, and remove async-await-pr…

…eview feature. (tokio-rs#1089)

The `async-await-preview` feature is removed as 0.1 will no longer track
Rust nightly.

This also bumps:
- tokio-timer (0.2.11).

Release tokio v0.1.20, tokio-timer v0.2.21, and remove async-await-pr…

…eview feature. (tokio-rs#1089)

The `async-await-preview` feature is removed as 0.1 will no longer track
Rust nightly.

This also bumps:
- tokio-timer (0.2.11).

trace: prepare tokio-trace for release (tokio-rs#1051)

Bump `tokio` to 0.1.19. (tokio-rs#1053)

This also bumps:

- tokio-async-await (0.1.7)
- tokio-buf (0.1.1)
- tokio-sync (0.1.5)
- tokio-threadpool (0.1.14)

Bump `tokio` to 0.1.19. (tokio-rs#1053)

This also bumps:

- tokio-async-await (0.1.7)
- tokio-buf (0.1.1)
- tokio-sync (0.1.5)
- tokio-threadpool (0.1.14)

Bump `tokio` to 0.1.19. (tokio-rs#1053)

This also bumps:

- tokio-async-await (0.1.7)
- tokio-buf (0.1.1)
- tokio-sync (0.1.5)
- tokio-threadpool (0.1.14)

Bump `tokio` to 0.1.19. (tokio-rs#1053)

This also bumps:

- tokio-async-await (0.1.7)
- tokio-buf (0.1.1)
- tokio-sync (0.1.5)
- tokio-threadpool (0.1.14)