message: Hide `tokio_postgres::client::SocketConfig` behind "runtime" feature
  This fixes warnings reported by rust 1.60 when doing the following:
  ```shell
  $ cargo test --manifest-path tokio-postgres/Cargo.toml --no-default-features
  ```
diff: use crate::codec::{BackendMessages, FrontendMessage};
  -use crate::config::{Host, SslMode};
  +#[cfg(feature = "runtime")]
  +use crate::config::Host;
  +use crate::config::SslMode;
  use crate::connection::{Request, RequestMessages};
  use crate::copy_out::CopyOutStream;
  use crate::query::RowStream;
  @@ -25,6 +27,7 @@ use std::collections::HashMap;
  use std::fmt;
  use std::sync::Arc;
  use std::task::{Context, Poll};
  +#[cfg(feature = "runtime")]
  use std::time::Duration;
  use tokio::io::{AsyncRead, AsyncWrite};
  @@ -145,6 +148,7 @@ impl InnerClient {
  }
  }
  +#[cfg(feature = "runtime")]
  #[derive(Clone)]
  pub(crate) struct SocketConfig {
  pub host: Host,
  