Upgrade various async dependencies

This upgrades tokio, futures & mysql_async to recent versions.  Doing so
involved reworking the async code in the tests to use the modern rust
async/await style, and updating code to work with the more recent
method names in mysql_async.

Author: mjgarton

Cargo.toml

@@ -32,7 +32,7 @@ time = "0.2.25"
 [dev-dependencies]
 postgres = "0.19.1"
 mysql = "18"
-mysql_async = "0.20.0"
+mysql_async = "0.29.0"
 slab = "0.4.2"
-tokio = "0.1.19"
-futures = "0.1.26"
+tokio = { version = "1.15.0", features = ["full"] }
+futures = "0.3.0"