Remove async_trait from the Handler trait (#8958)

Newest attempt to remove `async_trait` from the Handler trait.

Earlier attempts were in #7301 and #8296 .

libs/postgres_backend/src/lib.rs

@@ -81,17 +81,16 @@ pub fn is_expected_io_error(e: &io::Error) -> bool {
     )
 }
 
-#[async_trait::async_trait]
 pub trait Handler<IO> {
     /// Handle single query.
     /// postgres_backend will issue ReadyForQuery after calling this (this
     /// might be not what we want after CopyData streaming, but currently we don't
     /// care). It will also flush out the output buffer.
-    async fn process_query(
+    fn process_query(
         &mut self,
         pgb: &mut PostgresBackend<IO>,
         query_string: &str,
-    ) -> Result<(), QueryError>;
+    ) -> impl Future<Output = Result<(), QueryError>>;
 
     /// Called on startup packet receival, allows to process params.
     ///

libs/postgres_backend/tests/simple_select.rs

@@ -23,7 +23,6 @@ async fn make_tcp_pair() -> (TcpStream, TcpStream) {
 
 struct TestHandler {}
 
-#[async_trait::async_trait]
 impl<IO: AsyncRead + AsyncWrite + Unpin + Send> Handler<IO> for TestHandler {
     // return single col 'hey' for any query
     async fn process_query(