SendBatch now uses pipeline mode to prepare and describe statements

Previously, a batch with 10 unique parameterized statements executed
100 times would entail 11 network round trips. 1 for each prepare /
describe and 1 for executing them all. Now pipeline mode is used to
prepare / describe all statements in a single network round trip. So it
would only take 2 round trips.

pgconn/pgconn.go

@@ -1909,10 +1909,10 @@ func (p *Pipeline) Close() error {
 	for p.expectedReadyForQueryCount > 0 {
 		_, err := p.GetResults()
 		if err != nil {
+			p.err = err
 			var pgErr *PgError
 			if !errors.As(err, &pgErr) {
 				p.conn.asyncClose()
-				p.err = err
 				break
 			}
 		}