Fix data race in Postgres engine on connection close (#32650)

* Fix data race in Postgres engine.

* Use channel instead of `sync.WaitGroup`.

lib/srv/db/postgres/engine.go

@@ -418,8 +418,11 @@ func (e *Engine) receiveFromServer(serverConn *pgconn.PgConn, serverErrCh chan<-
 	copyReader, copyWriter := io.Pipe()
 	defer copyWriter.Close()
 
+	closeChan := make(chan struct{})
+
 	go func() {
 		defer copyReader.Close()
+		defer close(closeChan)
 
 		// server will never be used to write to server,
 		// which is why we pass io.Discard instead of e.rawServerConn
@@ -455,6 +458,8 @@ func (e *Engine) receiveFromServer(serverConn *pgconn.PgConn, serverErrCh chan<-
 		log.WithError(err).Warn("Server -> Client copy finished with unexpected error.")
 	}
 
+	<-closeChan
+
 	serverErrCh <- trace.Wrap(err)
 	log.Debugf("Stopped receiving from server. Transferred %v bytes.", total)
 }