Add flush() to front-end messages

The PostgreSQL wire protocol has a "Flush" message, which can be used by
the clients for long-lived connections. Add a flush() helper for it.

postgres-protocol/src/message/frontend.rs

@@ -271,6 +271,12 @@ where
     })
 }
 
+#[inline]
+pub fn flush(buf: &mut BytesMut) {
+    buf.put_u8(b'H');
+    write_body(buf, |_| Ok::<(), io::Error>(())).unwrap();
+}
+
 #[inline]
 pub fn sync(buf: &mut BytesMut) {
     buf.put_u8(b'S');