Update tokio requirement from 0.2.9 to 0.3.5

Updates the requirements on [tokio](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

foundationdb/Cargo.toml

@@ -54,4 +54,4 @@ num-bigint = { version = "0.3.0", optional = true }
 byteorder = "1.3.2"
 lazy_static = "1.4.0"
 log = "0.4.8"
-tokio = { version = "0.2.9", features = ["rt-core", "rt-threaded", "macros"] }
+tokio = { version = "0.3.5", features = ["rt-core", "rt-threaded", "macros"] }