refactor: tweaks after #3791 (#4022)

* restore fallback to `async-io` for `connect_tcp()` when `runtime-tokio` feature is enabled
* `smol` and `async-global-executor` both use `async-task`, so `JoinHandle` impls can be consolidated
* no need for duplicate `yield_now()` impls
* delete `impl Socket for ()`

sqlx-cli/Cargo.toml

@@ -55,9 +55,11 @@ features = [
 [features]
 default = ["postgres", "sqlite", "mysql", "native-tls", "completions", "sqlx-toml"]
 
+# TLS options
 rustls = ["sqlx/tls-rustls"]
 native-tls = ["sqlx/tls-native-tls"]
 
+# databases
 mysql = ["sqlx/mysql"]
 postgres = ["sqlx/postgres"]
 sqlite = ["sqlx/sqlite", "_sqlite"]